body work over old paint?

Generally you should take down to bare metal any of the areas that will need to be straightened or filled. And as long as the vehicle hasn't been re-painted before you're usually safe not stripping the whole thing. USUALLY is the key word. There is no uniform guarantee that old paint won't lift. - And the old paint can be hiding a lot of whatever might be underneath it. The surest bet is to know for sure what you're dealing with. If there are any indications of problems with the adhesion of the current paint, I'd strip it. If everything seems OK (no adhesion issues, not too many coats of paint already on the car, excessive chalking, etc...) then it comes down to trying to balance out the time/expense with just how nice you want the results to be. For a daily driver that's going to be used in winter, on gravel or as a hauler/grocery getter I'd probably not strip.