The shell has to be rust free and clean basically, there are a couple of sensible checks you can make, in order to check for rear panel damage, lift the boot lid and then look at the slam panel you should see a sticker from factory, lift the boot floor carpet you should find another orange sticker by the pre-lift pump access hatch, the drievrs side B pillar should also have another sticker from factory, lift the bonnet and look at the slam panel, there should be a couple of more factory stickers.
Check all the shut lines for any misplay, check the colour for any fading or slight differences in shades etc. But expect to have some paint work done due to the age of the car, stone chip repair, door dents etc. Any thing with signs of lacqer peeling then inspect further, if it doesn't tick all the boxes then walk away.
Front wings another little trick, where they meet the shell you should have a factory weld, almost looks like a blob of large solder same colour as the rest of the car, if not visible then the wing has been replaced, not problem if it has, as long as the job looks sensible, look at the inner wing see if it has a VW stamp on the replacement wing, if not then it's a patter part.
The key is the shell, if it's clean the rest can be sorted as part of your project, look at a few cars, we spent around two months finding the car we have currently, but I am very picky, it had to be a certain spec with original panels and we paid a premium for this experience.
Engine wise usual check on cold start, check the oil and it's condition (black, new or sludge) watch for any smoke on start up, let it warm up see how it all behaves during this period, check the gera selector for play and then check for gear engagement, how does the clutch and it's bite feel.
Take it for a test drive, check the coolant temp, check the MFA, check to see if it drives fairly straight when you let go of the steering, see how responsive the brakes are, do they feel spongy, does the car pull nicely in every gear and so on, few final checks, rear wash wipe operation, central locking behaviour if it has it, electric window behaviour if it has them, also if it has power steering then go from lock to lock to and look for any resistance or excesive noise.
Finally check the interior for any missing items, state of the seats, state of the carpet and the head lining, check all the swicth gear and it's operation, how mush history has the car/owner got & how long has the present/previous owner had the car for.
£2-3k should see a good example with many miles of motoring left in it provided it been looked after a little.
If you need any more then feel free to contact us.