4.0 Jeeps are bullet proof. Plenty of power and torque and extremely reliable. Some of the manual transmissions suck though. The Ba-10 used from 87-89 made by Peugeot is the worst Jeep gearbox ever created. The Ax-4 4 speed used from 87-90 is ok. The ax-5 is much better option used from 87-88. The ax-15 used from 89-97 is a great gearbox and easily swappable into the older models. As far as automatics go the aw -30/40 was used from 87-01 and is a variant of GM's 700r4 trans and is known to be reliable. My only other suggestion is check thoroughly for frame and body rust issues.