Hi Ian!
Welcome back; delighted to hear you're still on-track for the Big Tour, however it happens.
Add my vote to jags' and Andy's for the Sherpa as fantastic value in a heavy-touring bike. Notice I said "heavy-touring" rather than "expedition touring". The Nomad Mk2 is Thorn's current "expedition" bike in terms of outright load capacity, and is biased more completely toward that task, making it less of an all-rounder and less comfortable for general (non-loaded) use. It really comes into its element with massive loads, however, which is what it was designed for.
Keep in mind, the Sherpa Mk3's weight capacity is less than the Mk2, so the gap between it and the Nomad Mk2 is greater than before.
For all "normal-to-heavy" touring, I think the Sherpa is just ideal, and it works at full efficiency right out of the box, replacement parts are standard and readily available pretty much anywhere, and it is a wonderful all-'rounder when not touring, particularly if equipped with light wheels. When I was selecting a bike for my purposes, I had a choice of offerings here where I live. Co-Motion are located in the same town, and Surly's Long Haul Trucker is a popular choice stateside. Instead, I chose the Sherpa Mk2, which had a better design for my needs and was better suited for my purposes. If you don't actually
need or won't actually
use the Nomad Mk2's massive cargo capacity (I do and will to both), the Sherpa is a wonderful choice. Price-on-entry is less, it can be configured any number of ways, and can always be sold-on at minimal loss if you tire of it afterwards. Used ones are an even greater bargain and the absolute cost is well below that of a used Rohloff Nomad. Many have toured the world very happily.
I found my Sherpa Mk2 had a more lively, resiliant frame than my Nomad Mk2 has. With lighter wheels, the Sherpa Mk2 would be better suited for non-touring, sport-riding, and general use. This is even more true for the Sherpa Mk3.
The Nomad Mk2 is like the Sherpa Mk2, but "more" and might well be "too much" bike for those looking for an all-'rounder or who will have only a single bike for all purposes. Certainly, the Nomad Mk2 has a thicker-walled, larger-diameter (at both ends, since it uses non-conical tubing), less-resiliant frame than the Sherpa Mk2 (there's an even bigger difference between the Nomad Mk2 and the Sherpa Mk3) and depends more on wide tires run at low pressure to provide a comfortable ride.
So. A compelling case for the Sherpa as prime choice for loaded touring and general use; moreso for a used one. As Andy said...
In terms of bangs for your buck, I dont think there is anything that beats a second hand Sherpa.
Please keep us updated as your plans develop, Ian.
All the best,
Dan.