its simple things really.. typical "old car" problems like rust and just making sure all things work.
common rust places to check are the sills, bottom boot lip and wheel arches. if you decide to wait around for a completely rust free panda, you may have to wait some time or be willing to pay big bucks.
best thing to do is have a look at some pictures from a 4x4 restoration thread. (like my "Fallout" panda
photo album)
common faults with things with the 4x4 panda i have found are:
corroded brake lines
electronics not working right. mostly down to simple earthing faults
CV boots perished so all the gearbox oil has leaked out
the central prop shaft can come loose from the mounts and CV boots be perished
shock absorbers leaking
damaged rear leaf springs (sometimes missing half of one of the 3 leafs)
door locks can fail if they are old
if the car has been standing for a long time, be sure to give the brakes a really good looking at. both of my 4x4s had to have a lot of the brake components replaced.
engine-wise they are pretty bomb proof
be sure to check for signs of head gasket failure also the condition of the cam belt. not that it is a hard job to replace yourself. these engines also suffer no damage when the cam belt snaps which is very nice.
you can sometimes get rough idle but that is often down to the vacuum servo on the distributor failed causing an air leak and incorrect ignition timing.
(no matter how bad the electrics seem to be with earthing issues, the engine always seems to run - maybe thats just me though)
oh.. a crunch when shifting to 3rd gear is not uncommon. mine does it as it does with other forum members panda 4x4s.
i think i covered just about everything.. if i remember anything else i will reply back with it.
hope this helps!