Wow... some of those questions would take pages to answer...
But here goes.
Sammies are as reliable as you (the owner/maintainer) make them. If you take care of them and keep them tuned up they will run forever. But they aren't big block chevies, so don't expect alot of power unless you upgrade a the motor a bit.
In the sand they can be made to do great things! They are very light, and with some wide tires and a bit of lift and a locker or two you can stay on top of the sand and make others look silly.
Depending on the age/condition of the drivetrain, and depending on how big of a tire you are running, you can get between 20 and 35 mpg. That seems like a big gap there - but remember these came from the factory with skinny 27" tires and most folks out there tend to push that up a bit. My 'little' Sammy is running 31x10.50's and I get about 28mpg.
The Samurai has a 10 gallon tank, but when you hit 'E' you still have almost 2 gallons in reserve. Here in the US you can still get a 16 gallon replacement from PetroWorks
that will add to your range.
The Samurai (US model) came with discs up front and drums out back, but there are kits to upgrade the rears to discs.
Problems to look for?
-Rust at the fenders, body mounts and floor
-home-made suspension components (can get scarry)
-home-made steering systems (even scarrier)
The one you linked to looks decent, but I would price that one more around 2-2.5K at the most. They really haven't done much to warrant that large of a price tag (the winch looks like an ATV model - HA!)
Look at more than just one or two before you buy. And if you can find one, buy the newer 90-95 model where you will have EFI from the factory (less headaches).
Ok, anyone else want to toss in a few - my fingers are tired...