On top of looking at the condition of the fluids, like you would any other used vehicle. Take a good look at the workmanship of what has been done to it.
How the motor mounts are welded in, if there is wiring dangling all over the place, leaks on the drivetrain that hasn't been addressed.
Just to get a feel for if it's a cobbled up POS or not.
I used to bring stuff into my shop like this and make it right, it can be an expensive, frustrating and lengthy process.
I looked at a Samurai a few years ago that looked great from the road.
Guy wanted $2500 firm for it. It had a 2.8L Chevy V6 in it
After 5 minutes of looking at it I offered $500 and the only thing I could have salvaged was the body.
The amount a hack work done to that poor thing was criminal.
I still see it posted up on CL at least once or twice a year, some poor guy buys it figuring he can fix it and just gives up.
Take a good, hard look at this thing. Bring a professional who will give you an honest opinion on it.
Buy your professional lunch and a couple good beers afterwards, because he can save you a lot of
If you find a few things that don't look right or look slapped together, walk away. Or be prepared to take the whole rig apart and start over to make it right.
I know guys who are professional turd polishers and have no scruples about selling someone shiny junk to make a buck.