these are a few things i learnt along the way when i buy car's,
start up car, run at idle for about 5-6 min's, carefully remove the oil fill cap on the engine, if oil squirt's out, dont get it, blown piston rings
with car on, look at temp gadge, if it's still cool, it's good. turn car off
remove radiator cap, if it's hard to remove the presure is all wrong, dont get it, cooling system problem, could be a faulty water pump or a thermostat. if their is any bubbles in the water/coolent, dont get it, blown head gasket.
remove dipstick, if oil is black or brown it's ok, (black just means need's oil change) if it's cloudy or milky white, dont get it, blown head gasket.
if it's auto remove the dipstick for auto fluid, the oil should be pink or a slightly colored red. rub hand down stick, if their is any black residue on your finger's, dont get it, bad gearbox.
look for the usual gearbox, engine, diff leaks. listen for the usual rattles from engine bay, dont worry about pinging sound, that's just bad fluid. secondly, drive the thing, if you have any doubts, ring the automotive club of your state, we got racq in queensland. they will put it over the pit's and give it a good check.
hope this helps