Read the NOOB guide
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=54207@ 110000 miles, Check for maintenance records (actual shop receipts) of a recent timing belt change.
If this is not available, you
must assume that this needs to be done as immediately as possible. (~$600 + labor)
This needs to be done every 6-7 years or 80-100k miles.
It is imperative that you know the age/mileage of this part. Replacement is expensive but Failure is
several times more expensive.
If it has an in-tank lift pump, this will eliminate MOST driveability complaints/issues. If it doesn't have one, you should get one. While the engine is idling, check the hand pump plunger on the housing of the fuel filter head (next to the brake booster). If this can be depressed with firm hand pressure, then it probably doesn't have one, and if it does, it's not working.
While you are there, check the wire connectors plugged in to the front of the fuel filter housing. If one is blue and has wiring splice connectors within a foot of the connector, then it has the upgraded fuel filter head. If not, then this will need to be upgraded, as the original poses a significant fire hazard.
Look at the intake side (battery side) of the engine (behind and below the alternator.) [A flashlight helps] If the factory EGR system is still there, then it should be disabled asap to prevent further damage / problems.
Another good thing to look for is a Provent or equivalent catch can system for removing oil vapors from crankcase gasses. This is good for preventing this sludge from mucking up the whole intake side of the engine.
These are some of the important issues unique to the CRD Liberty, in addition to all the normal stuff one needs to check when buying a used vehicle. One cannot stress enough, the need for actual physical receipts for parts/repairs/maintenance.
No matter what is claimed, if you cannot see the part, or the receipt for the part, assume it didn't happen / does not exist.