Welcome!! I have a TDI Golf for mileage (about 50 mpg) but I usually opt for "MONGO" my 1009 instead cause it's just cool and fun to drive. I don't quite get 25 but more like 19-21 mpg but Mongo does get better mileage than my 2005 Caravan though.
As far as what to look for, that depends on what your going to do with it. If it's going to be a daily driver pay attention to the engine and trans (rusty body parts can be changed fairly easily and cheaply - an engine or trans will cost you if they go). Check for leaks on the ground under the engine, trans and diffs. Look for anything wet on the underside. Try to hear a cold start so you can listen for any unusual sounds (these make MANY weird sounds anyway) out of the ordinary. Look at the idiot lights when it starts, all should light up AND GO OFF after it starts. After it warms up a bit take it for a drive and really pay attention to the shifting of the tranny. Feel for slippage through the gears. Test 4 High and 4 Low to ensure the transfer case is in good shape. Check the steering for wander, pulling or binding. This could lead to MANY bad things. Check the brakes for squeaking, pulling, sponginess etc. Do a "Panic" stop to see if it pulls to one side or another.
Depending on how mechanically inclined you are, you may be able to fix many of these issues yourself. But if you're not, be prepared to learn or pay someone to fix the problems. These are just some of the BASIC things to look for. When I go look at vehicles I take my time and look them over carefully so I know exactly what I'm getting. The thread mentioned above, "What have you done to your CUCV" is a good read to hear what others have done or plan to do.
Good luck and welcome from Illinois!!