OK, got you. You´re looking for a comfortable, supersonic dune buggy, which isn´t stressed at 1ton payload to keep up with bikes for a fist full of change.
I´d still trust that UAZ.
Reliability of that thing is relative. Maintainabilty is for sure. At this price it is a steal. You could iron out a lot, for what you still have on your budget and fuel up for most of the trip.
As s´one already mentioned well cared firetrucks. How about this one
MAN 12.192 4x4. or this
IVECO?
The MAN has already a 1200l water tank built in and lot´s of space to carry all catering and needs everywhere. Not quick, surely a little bit thirstier than a car. 130hp Ivecos start at 4k€, so you could save a lot for fuel upfront.
Too big. Here is one size smaller. Definitely does the job:
MAN VW-MAN 8.150-FAE
But still 90km/h max and 20l/100km (of all qualities) of Diesel.
No Truck, too slow and thirsty?
No car (130, LC, etc), with 1ton always at it´s limit?
So, vans? But, I´d guess you do not want to bring a plastic electronic
Sprinter to the dunes, when you already mistrust a mechanically governed 2.8 Isuzu?
In the van class, my all-time favorite definitely is a´90s MB410D with Iglhaut 4x4 and decent tires. Here is a similar well maintained low milage
309D 4x4. Not sure, if a Iglhaut with G-Model axles. These are rare, and sought after. That´s why not exactly in the budget. As this
VW LT40 4x4
Really curious, what you will end up with.
Good luck.