When buying a 4runner or any car I go by this checklist..
1)First thing is the simplest, ask current owner for maintence reciepts. Look for headgasket, timing belt/water pump maintenence. oil change maintenence (Those 4 are current issues with the 3.0) If its a 22RE motor, ask for Timing chain maintence receipts.
2) Check the Fluids. Check the oil, make sure it has oil in it #1, make sure since its an older motor, that the oil should be black and not chocolate milk looking. Chocholate milk looking is signs of a blown headgasket. Check the Radiator make sure it looks like fresh water/antifreeze, not brown dirty water or crap floating in it. If its an auto tranny pull the dipstick and look smell it, if it smells like burnt toast, but looks good, signs of transmission problems trying to cover up. Smells bad/looks bad=gpnna be bad, might not be now, but soon.
3)Start it and take it for a test drive. Let it warm up and watch the gauges. Make sure it idles and doesnt have vaccum leaks.
4)ask for current title / registration / or SMOG Papers. Here in California the rules are so strick, that if any vehcile passes smog, its usually a good sign of a good running vehicle.