Are the Sherpa and Raven still available? I was under the impression that they had been dropped from the Thorn range, so for heavy duty touring it is now the Nomad in either Rohloff or derailleur version.
But there might still be some old stock Sherpa and Raven frames. One point is that AFAIK neither Sherpa or Raven can use disc brakes, whereas the current Nomad, Mercury and Club Tour are designed to be able to use these.
For lighter duty touring, AFAIK it is the Mercury if you want a Rohloff or the Club Tour for derailleur.
And for unloaded day rides on tarmac a light build Mercury for Rohloff and either a light build Club Tour or an Audax for derailleur.
This has changed from when I got my touring Thorn eight years ago. At that time the Nomad was recommended for out and out expedition touring or for touring involving a high proportion of off-road use. But for my projected use of cycle-camping in Europe with occasional use of tracks and paths Thorn advised me to get a Raven instead, with heavy duty wheels and tyres.
In the event I went for a clearance Raven Tour, which was slightly cheaper and slightly heavier-duty than the more recent Raven model but lighter and not as stiff as a Nomad. It has been plenty good enough for me.
For your dream use, I suppose the Nomad would be the best current option, especially if you want disc brakes. If you can find one, a clearance Sherpa or Raven with heavy-duty wheels would IMO be a good option and more pleasant to ride unloaded for "normal" use. If you don't mind buying second-hand, a Raven or Raven Tour would be good options, as, contrary to derailleurs, the Rohloff hub generally improves with use.
Whichever (Rohloff) bike you chose, a Chainglider is a possible alternative to belt drive. I was sceptical at first, because I thought a free-floating chaincase would cause friction and noise but I am now a convert. Chainglider is a cheap way of getting the low maintenance and cleanliness advantages of a belt drive while conserving the ease of replacement of a standard chain, which should be easy to find in most countries.