Thanks again for all the great info.

I checked with ICE Recumbent Trikes (here in the UK) because they sell a modified Rohloff shifter which is extended so that the entire grip rotates.  HP Velotechnik also supply similar (it could be the same product).  On a recumbent a normal twist shifter would end up being positioned so that you'd have to grip it with your little finger and heel of your hand, which I imagine could be awkward and tiring.  The extended shifter allows you to grip and rotate the shifter with thumb and all fingers.  I think it's over £200, though. 

I always thought that buying a used bike with a 'run-in' Rohloff would be a good option, but I think in this case it would make more sense to buy new. 

I'll check out the various Rohloff fittings.  Although the internal mech might be fine for this bike, I suppose it could be wiser to buy a hub which has the disc brake option and external mech, as there'd be more chance of it being compatible with a modern frame if I upgraded the bike.

Decades ago most bikes did not have much in the way of brazed on cable stops or cable guides.  Even water bottle cages came with straps to strap around the frame.  Back then, bolt on cable housing stops were made that would strap around the chainstay for the rear derailleur cable.  My point is that this could be another option if you can find such cable stops for the shifter cables if the canti brake mount bracket does not work out.  Attached photo shows an example, this is on a 1960s vintage bike I have not ridden in years that I have in storage.

On gearing, if you could figure out what the lowest gear that you would need for the steepest of hills and pick sprockets and chainring to match that for first gear, that might be the way to go.  I knew before I bought my Rohloff bike and hub that I would want two sets of gears, one that was ideal for around town riding near my home on an unladen bike.  And a different set of gears for heavy touring where my lowest gear was my priority.

On my other bikes, I noted what derailleur gears I used for the extremes of uphill and downhill, picked my chainring size based on that for my riding around home.  I decided to stick with the standard sprocket size of 16.  I am a retired engineer, so I did this with a spreadsheet.  But now there are gear calculators on line that would make that easy.  These are the gears I use around home.
https://gear-calculator.com/?GR=RLSH&KB=44&RZ=16&UF=2120&TF=72&SL=2.6&UN=MPH&DV=teeth

For comparison, these are the gears on my Thorn Sherpa touring bike which has a 3X8 half step plus granny drive train, I wanted my Rohloff gears to be similar to these gears for unladen riding.  I try to avoid using the two most cross chained gears on each chainring, so I set it so that those two gears are in light gray instead of black on the graphic.  But you with a recumbent, not sure if cross chaining is a thing or not.  A friend of mine had a recumbent trike with a 3X10 drivetrain, cross chaining was not an issue.
https://gear-calculator.com/?GR=DERS&KB=24,42,46&RZ=11,12,14,16,18,21,26,32&UF=2200&TF=72&SL=2.2&UN=MPH&DV=teeth

And for heavy touring, I wanted a first gear that would give me a cadence of 72 at 3.5 miles per hour, which is about the slowest speed I can pedal without a lot of over-steering to maintain vertical and directional stability.  And I picked a chainring that would give me that for my lowest gear.  I swap chainrings and add or subtract four chain links when I change chainrings.

You could play around with options at that website.

Or, plan B.  Contact some recumbant bike builders that have fitted their bikes with Rohloff hubs and ask what sprocket and chainring they used.

I assume your knowledge of Rohloff is not very high.  So, you may not know about cable tension.  You want a lot of slack in the shifter cables, this is the opposite of derailleur systems.  If I am in gear 10, my shifter can turn from 9.5 to 10.5 by just taking up the slack in one cable or the other.  If your cables are too tight, that can lead to gear shifting problems.  That can be a hard thing to get used to when you are used to derailleur systems that have no slack in the cables.

You will have to figure out where to put your shifter.  I am clueless about the controls on your recumbent bike.  The Rohloff shifter is sized for the end on a mountain bike handlebar.  There are lots of other options.
https://www.cyclingabout.com/rohloff-hubs-with-drop-handlebars

Second attached photo, I use the Hubbub adapter to put the Rohloff shifter on my drop bar handlebar end.

I see once again PH posted a reply while I was typing.  Nothing I wrote conflicts with his comments.

Thanks, everyone.  That's really helpful.

I've watched a video on changing the cables for the internal mech - it doesn't look too bad if you change them at home as part of routine maintenance.

I did notice that they had a 2-cable stop which bolted into the canti brake studs.  I'll look at my bike, but that might simplify cable routing.  It's an underseat steering longwheelbase bike, so the cables could be run from the handlebars under the seat to the 'seat stay' where the brake posts are, and from there the inners would run down to the hub, so I don't think I'd need anything along the chainstay.  I think it would be a relatively short cable run, too. EDIT - I've just had a look at the bike, and the rear brake (an old Magura HS hydraulic rim brake of some sort) is on the underside of the chainstays.

As Tigerbiten says, chainline probably won't be too much of an issue due to the long chain on this bike.  I'm not sure if it would need any sort of chain tube or idler, or if it would need some sort of chain retention at the chainring (eg a derailleur cage or a narrow-wide chainring).  Perhaps the chain tensioner and a correctly sized chain would keep things under control.

Gearing might take a couple of goes to get right, but I'd probably try a larger rear sprocket, say 19T, with a larger chainring of around 48T, 50T or 52T.  I suppose the thing is to gear it such that the majority of your riding is in the most efficient gears.

As I said, it's early days, but if I decide to give it a go and bought a new hub I'd get it from SJS Cycles and have them build it into a wheel so that the build would meet Rohloff's recommendations.  Speaking from experience with other recumbent wheels, broken flanges, snapped spokes and split rims are bad!

The bottom bracket will have to be the last thing you order, because the Rohloff chainline is expected to be within one millimeter of true. (I know, several parties have skewed chain lines and claim no bad effects.) A straight chainlink helps reduce noise and increases chain/sprocket life.

If you're not planning to fit a Hebie Chainglider (it's a near-complete chain enclosure, the nearest thing to the mythical oil bath for the transmission), you could buy a secondhand Thorne chainring, a default fitment that is good and thick, very long-lasting, but for the same reason taken off by cyclists fitting the Chainglider because it requires a thinner chainring. You might be able to pick up the Thorn chainring at a good price on the forum.

Gotta go cook.

First of all, one topic you did not mention is spoke pattern.  Make sure that the wheel builder reads the Rohloff instructions for wheel builders.   I had my bike in a bike shop, a mechanic was looking at it and said that he was going to build up a Rohloff wheel for a customer.  Then he said he was going to use three cross, but was curious why mine was two cross.  I asked him if he had read the Rohloff instructions for the wheel builder, he said he had experience and did not have to read anything about building wheels.  I then told him if he built up the wheel the way he planned, his customer might have a lot of spoke breakage.  I have no clue what he did.  Point being, if your mechanic says he does not need to read the Rohloff instructions, find a different wheel builder.

I built up my wheel over a decade ago, I think now Rohloff specifies Sapim spokes where they did not when I built mine.  Sapim Polyax nipples, I would suggest them.

The Rohloff puts a lot of torque on the frame, so you probably need the hub with that long torque arm.

My bike uses the EX box.  It is easy for me to fit new cables.  I am clueless about the internal cable, so can't comment on that. 

I see no reason you can't use a couple zip ties for cable guides.  But if you had the kind of hub with internal cable, you would need some cable housing stops. 

Yes, 135mm.  I suspect that the nutted hub would work, but I would go with the quick release.  Hub does not come with a quick release, you would need to add the skewer to your shopping list.

I used 36 spoke for my bike, it is used for heavy touring.  For your purposes, you may prefer 32 as some rims are getting hard to find in 36.  But if your rim is readily available in both 32 and 36, use whichever you prefer.

You did not ask about gearing.  You need to figure out what gear range you want so you can chose the chainring size for your crankset.  I use a 44T chainring for riding near home on an unladen bike, but I change to a 36T for loaded heavy touring.  This is the Thorn forum.  Thorn uses 17T sprockets by default, but Rohloff comes with 16T. 

You did not mention chainline.  I think there is some info on that in the Rohloff printed materials, but I built my bike over a decade ago, I do not recall the details on how I picked bottom bracket spindle length.

I see PH just posted a response.  I agree with everything he said.