The U100L you're considering is a nice truck and the owner, Greg, is a good guy. In standard form, the U100L is a bit more modern and comfortable than the standard U1300L but leather seats and A/C are easy additions. The U100L is quieter than the U1300L and far more like a US pickup. It's smaller so it's easier to manoeuver, will consume less fuel, pollute less (Euro II emissions), and will be less expensive when it comes time for tires (typically much sooner than you might expect as they don't last on the big trucks). The trade off is that it's not quite as good off road as it's bigger cousin due to the lack of a really low crawling gearbox and less ground clearance. Unless you really plan to push it, this may never be a problem. Also depending on where you plan to take it, the U100L engine is managed electronically, the U1300L's is all mechanical and the OM352/352A engines are one of the most common and robust diesels in the world.
Another thing to consider is that until you have the camper body on it, you'll need to stop at the scales with the U1300L but not the U100L as its GVW is 5500kgs. Both trucks have air over hydraulic brakes so you'll need an air endorsement on your DL. As Charlie mentioned, weight adds up quickly and you should stay well below the max GVW for off road use or you'll be breaking and repairing things often. If you can live with the lower GVW, the smaller truck may be the more sensible option. I like the U100L but prefer my U1300L37 for my needs.
If you do decide to go for the U100L, the original fiberglass camper body that was built for that specific truck is still floating around. In fact, I believe a member of this forum mhiscox (Mike Hiscox) owns (or owned) it and may be able to help you to reunite the two...
Yep, the U100 was quieter and more comfortable that the U1300L. However, not by as large of a margin as I would have thought. Also, as you pointed out, A/C and nice leather air-ride seats would make it an even closer contest. The U1300L I saw had an ex-military radio box on the back which must have been quite heavy. Inside it had a fridge, table / bed, shower, sink, water tanks, battery array, toilet, etc. so was carrying a fair bit of weight. In spite of that, it was much livelier than I thought it would be. Although it wouldn't cruise down the hwy at 100 km/hr like the U100, I think it would still be fine.
In terms of off-road performance, I probably won't really know what I'll need until I'm actually out on my trip. However, at this point I'm thinking either vehicle would probably meet my needs. The increased capability of the U1300 is really nice, but then again, the smaller size, comfort and better fuel economy of the U100 is also a draw.
The fiberglass body that used to be on Greg's truck looks very nice and by the way, Greg is a very nice chap. How do you think a fiberglass body would stand up to the rigors on a long multi-year journey as opposed to one of the military radio boxes? It would obviously be much lighter and easier to carry around, but I'm not sure it would be a tough as the radio box. However, that's just my gut feel and not based on any previous experience with either type. Thanks for Mike's info. I'll try and get in touch with him and see if he knows where the body is these days.