I would certainly consider the Mercedes. As you stated, my biggest concerns would be parts availability and serious lack of power.
Terry Lee Enterprises (Rob Pickering) can get you anything you need. Bill Caid has been operating his 1017A in the US for 7+ years now, big trips every year and no major issues getting a part (unless they were being shipped through Texas during a Hurricane).
On the 'surplus' firetrucks, most have less then 30,000km, so probably only the 'rubber' needs changing due to age, and perhaps some things in the air system. But also the fire departments keep up on the maintenance, my truck with <30,000 km has had most of those items replaced anyway.
They are never going to blast up a hill (but neither will a Fuso). I think the 'axles' are the most important thing. Mine has fast (4.8) axles and with 395/85-20 XZL (46") it will cruise in the 55-60 mph range and can hit 65-68 mph (by GPS) with an OM352A-(168hp) at 15,000 lbs. The XZLs are only rated to 55 mph anyway. My 2002 Fuso FG with 37" tires cruises a little faster, but I don't like it much over 60 anyway. The newer Fuso (post '07) are hot rods compared to that.
Once you are in the 240hp range (as
Joe917 pointed out) either by buying a truck with that engine rating to start, or 'tweaking' (add an intercooler, 'adjusting' the pump, 5 nozzle injectors, etc) I think you are in the sweet spot. There are plenty of MB trucks overseas with 240 to 320 hp stock.
Yup, they will never be
'Merican high horsepower monsters, but no Cabover 4x4s for us.
And yup! I think there is a market for 'converted' cabovers. Inexpensive 2x4 Fusos, Isuzus (and Chevy version), etc, converted to 4x4 with an American powerplant and American axles with all of the aftermarket goodies you can buy for them (power upgrades, lockers, suspension, etc), would be a nice turn key vehicle. It sounded like Rebel Offroad was going down that path, but it seems like it died ? (
http://forum.expeditionportal.com/threads/144320-Isuzu-NPR-4x4-Beast!-Rebelzilla-by-Rebel-Off-Road )
