I would just look at the frame, tends to rust in the rear like you stated. I have noticed it always tends to be the driver side floor board that is rusting out. I would look for any oil leaks, the rear main seal is a problem child for the jeeps. Also just look over the engine bay. Check out the radiator. and thermostat housing for leaks. Push down on the rear and the front of the vehicle. Checking for the shocks.
After buying the vehicle, i would just do a fresh tune up. Spark plugs, wires, distributor, oil change, and i would do the tranny and diff fluids also. Just so YOU know when everything was replaced, not HIS word.
I bought my jeep for $1500, some minor body rust. some rust on the frame rails. I had to replace the radiator, water pump, and the thermostat (pretty easy job, parts weren't to expensive, maybe $400 with fluid?) and now the slave cylinder is leaking (again, not to expensive fix, just takes a lot of time removing the tranny etc etc.)
Okay. That is everything i am gonna say
haha. Goodluck! Have some FUN! Jeeps are so amazing, cheap parts (can find them anywhere) very easy to work on!