They are fairly simple little handguns so there's not much to damage that isn't immediately apparent. One thing I have noticed is that the hammerblock is sometimes lost by careless disassembly. It sits in the frame just forward of the hammer, you should consult a good diagram to figure out which model of the PPK you are examining, and where the block should be. This is especially important if you want to carry the gun for self-defense.
A simple test for this is to see if the hammer can be pushed all the way forward if the trigger is not all the way back.
Otherwise, if it's in generally good shape all you may need is a spring service pack from Wolff Gunsprings, and it's as good as new.
If the gun is fairly old, you might want to consider new grips - old plastic is prone to splitting.
Parts and magazines are available from a number of sources.
I've carried one for three years now. It's light, comfortable and easy to conceal. Mine was made in 1921 and except for some pitting on the slide, still looks new. I've never had a feed or ejection problem with it and I've found it to be quite accurate. For carry purposes I use the imported Fiocchi hollow points. I've tested these on deer carcasses and they do the job quite well. Fiocchi also makes a hardball load if HPs are prohibited in your area.