Jim already told you about the rims. Keep in mind that the car you're going to look at will have 6 volt electrics - unless it has been converted to 12 V .
This is not a big deal, but does make finding electrical components a little more difficult. Rust is the thing to watch out for in all old cars and it's thetrickiest thing for an amateur to repair and a big money-eater if you have it done professionally. On the other hand, the PV is put together very simply, and many major body components can be replaced by simply unbolting them and bolting in new ones, which are generally still available. Floor boards are frequently rusted through, particularly on the driver's side, but repairs with plain sheet metal are not very complicated. The pseudo frame pieces beneath the floor and side rails are harder to deal with, so carefully check their condition. Even if the car doesn't run, remove the plugs, squirt a liberal amount of oil into the cylinders and try to turn it over with a wrench on the crankshaft bolt -- 13/16 socket -- hardly anything metric in this car. If you can turn it over, there's a good chance you'll be able to get it to run once you get it home. I wouldn't try to tow it but would rather load it on trailer. But if you must tow, disconnect the driveshaft from the differential to protect the gearbox - you won't know what is in there and an easy repair job can get complicated if there are loose pieces of metal floating or rattling around in there. If it's really a $300 car, expect to spend at least 10 times that to turn it into a safe driver, 20 times to make it nice, 30 times to make a show car. But you don't need to be a mechanical genius to be able to just about everything, except for engine machining and finish body work, yourself. These cars are not just nice conversation pieces, they are fun to work on and a ball to drive. Keep in mind, though, that ones in pretty good condition regularly come up for sale for fairly reasonable prices, and that to turn a "parts car" into a driver often costs quite a bit more than buying the equivalent. Remember the old saw about never recovering what you put into and old car when you sell it. So, let the other guy pay.
Good luck,
Bob S.