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.

It's early stages, but I'm considering installing a Rohloff on an old recumbent bike.

The bike has a fairly standard rear triangle made from slender-ish steel tubes.  Vertical dropouts, and 135mm rear spacing (current hub has a quick release).  Rear wheel is 700c, and it has rim brakes.  It also has a rear derailleur hanger which could be used for a Rohloff chain tensioner.

There are no cable guides along the left hand (non-drive side) chainstay.

From what I see I'd need a 'regular' 135mm hub with either the disc brake cap (for any future changes) or smooth cap. 

Also, would either the QR or threaded axle work - is there any preference?

I'm not sure about the internal or external gear mech.  Which would work best on a bike which wasn't built with Rohloffs in mind?

I expect I'd also need the 'standard' hub which uses a separate torque arm.  Is this likely to cause any problems?

Excellent condition. Ridden about 200 miles since new. I'm 5' 9 1/2" tall. Deore Hollowtech chainset, SRAM X-7 rear derailleur (9 speed), Deore front, Ergon GP5 grips, Deore brake levers, SRAM X-7 shifters, Avid brake callipers, Brooks B17 saddle, Thorn seat post, Shimano 105 rear hub, Deore LX front hub, Sun Rhyno rims f+r (upgrade), Schwalbe Marathon Plus tyres. Price does NOT include bar bag, bottle holders and bottle. I might consider selling these as an addition.  Some measurements: Actual top tube 560 - virtual top tube about 575
Location: Wirral £550