Differences between 700c Nomad & Mercury

[thenandrins]

Background
I own a lovely Mercury. I bought it because I do long distances (audaxes) and got fed up with buying Shimano parts. I also just fell in love with the idea of a Rohloff and wanted to try one out. I run it with caliper brakes, a carbon fork and 28mm 4 season tyres. No rack - I use a saddle bag. I'm certainly not a weight weeny, but it is on the light side.
I did not sell my steel audax with Shimano Ultegra and as it turns out, I have naturally gravitated back to it for long, fast rides.

After quite a few audax rides, I have decided - concluded - that the light Rohloff set up is not really what it is best at. I really think Rohloff is for touring.

Touring wise, I do not take the kitchen sink. I tend to either have fairly full rear panniers or I split the load over 4 smaller panniers, one at each corner. I have a Hewitt. This is also lovely to ride, loaded and unloaded. I run 35mm on it at present. Road touring 99% of the time - perhaps some "loose sandy" type bridleways - not like CyclingAbout man!
As an audaxer, you can imagine, that even when touring, I do not ponder. I do get some miles in each day.

Additionally, disc is everywhere I am have a - admittedly slight - concern that my fleet is aging and parts will be harder and harder to find.

So, my question is -
Should I sell my Mercury AND Hewitt and buy a Nomad disc.
Or
Sell my Hewitt and with the cash, upgrade my Mercury. I should re-rim it (currently Ryde Chrina), buy disc calipers front and back and a new fork then add the disc to the hub. I will need a new SON at the front ... I note that SJS has a disc fork for a Mercury (not sure I've seen a Mercury with a disc at the front?) 
https://www.sjscycles.co.uk/forks/46-700c-thorn-disc-steel-fork-blue-imron/?geoc=BE
If I were to do all this, would my Mercury to be a tourer?
is 853 good for touring? Does this make a difference?
I think either option would cost about the same.... anyway, this is not about this.
(Not bothered about lack of parts at present - this can wait. I am also not worried about the hassle of it all.....as long as I'm happy with the result)

Put another way - what do you think the riding difference is between a 700c Mercury disc and 700c Nomad disc?  The geos? My Mercury is 610S.

Thanks a lot for your comments and advice. This topic has been rattling about for a few months and it is starting to drive me mad!

[WorldTourer]

The Nomad frame is very heavy and rugged and it is overkill for anything other than true expedition touring – and even if you are an expedition tourer but aren’t a heavy rider, it is still more than you need. My partner recently built up a touring bike on top of a Surly Troll frame with the exact same components as my Nomad Mk2. The Troll is already a heavy frame suitable for expedition touring, and yet I am amazed at how much lighter her result was than my Nomad.

Of course you could buy a Nomad if you are set on a specifically Thorn frame, and the Nomad is the only one close enough to your needs. But if you search outside of Thorn's products, there are better options.

[PH]

That's an expensive set of questions and despite being familiar with the bikes I'm not sure of the answers.  I think a fair bit of it depends on you, your weight, riding style and preferences. 
I like Audaxing on my Mercury, so much so that I sold my Ti Audax bike, the Merc is a bit slower, but I treat Audax as a fast tour, I'm naturally closer to the minimum speed than the maximum, though I can't think when I was last concerned about being out of time.
I don't like touring on it, my lightest camping load puts me over 100kg and getting close to the bikes recommended maximum weight, with the 853 fork.  If you get out of the saddle it can feel like the tail is wagging, a 20kg lighter rider might have a different experience.
I also have a Hewitt, a Cheviot SE, it's a lovely bike and I don't ride it! It falls between the Mercury and another, I keep meaning to build it up differently to make it more distinguishable, I'd sell it, but despite it being pristine they fetch peanuts...
I built my Mercury up from a frameset, moving the bits over from another bike, I'm not sure it was the right decision, by the time I'd bought the none compatible bits, and had the hub converted to disc, I hadn't saved much over buying new and selling what I had.

OK, here's what I would do.
Put a steel V brake fork in the Mercury and go touring without changing much else, if you're lucky the Hewitt fork will fit, if not there's plenty about and they're not expensive.  If you get on with that, do the other stuff, new wheels, disc brakes, hub conversion...

If you're not happy with that:
Put the Rohloff in the Hewitt, it'll need a tensioner and a long Torque arm and you'll have to zip tie the cables, Paul Hewitt has built a few up like this. If you like that, then I'd consider a new frame/bike or having a frame builder make that neater and more permanent. 

Both of those options involve buying stuff that you're not going to keep, but I think it's a small price to pay for going into the long term changes better informed.

[PH]

The Nomad frame is very heavy and rugged and it is overkill for anything other than true expedition touring

Have a look at this build:
http://www.sjscycles.com/thornpdf/BUILD7NOMAD700ABSPECIALROHLOFF.pdf
That brochure says:

The resulting machine is almost as light as a 700c Mercury with identical 853 fork

[JohnR]

Perhaps read through the Thorn Bible http://www.sjscycles.com/thornpdf/thorn_mega_brochure.pdf . You can get a Mercury with a front disc brake - see picture on page 2 (it's now my bike ) - while the table on page 23 compares the suitability of different models for different conditions.

[thenandrins]

Thanks for everyone's reply.
I need to think about the rim/tyres. If I can keep the rim, I can change the rear to disc easily and simply swap out the fork at the front. Thanks PH for this. The fork could also have a rack and therefore I can run it as a light tourer. This fits with the usage table - Thanks JohnR.
WorldTourer - yes, Nomad it probably overbuilt.

PH - as a side note, it is weird that Hewitt's have a poor - very poor - resale value. Surly and Thorn both go for not that far off original prices whilst there is little interest in Hewitt, yet the Cheviot SE was adored by Chris J. It is a great bike.

[PH]

PH - as a side note, it is weird that Hewitt's have a poor - very poor - resale value. Surly and Thorn both go for not that far off original prices whilst there is little interest in Hewitt, yet the Cheviot SE was adored by Chris J. It is a great bike.

I'm not sure.  I've also seen some other traditional tourers fetch peanuts in the last year or two, including a few Thorns. I think fashion has moved on, Hewitt is no longer doing a touring frame and even Surly have dropped the Long Haul Trucker.  Of course the measure of peanuts isn't precise and may differ, for me it would be anything less than a third of the original price, if in decent condition.  There's a nice looking Cheviot SE advertised on another forum, younger than mine, and I wouldn't part with that for their asking price.

[mickeg]

...
Additionally, disc is everywhere I am have a - admittedly slight - concern that my fleet is aging and parts will be harder and harder to find.

So, my question is -
Should I sell my Mercury AND Hewitt and buy a Nomad disc.
Or
Sell my Hewitt and with the cash, upgrade my Mercury. ...
...

Wait until parts are so hard to find that you know for certain that it is time for a new bike.

My newest bike is only three years old, rim brakes, I am very happy with it.