if its made of metal it is probably rusty.
from my experience I would check the following:
- boot floor - lift up the carpet and check, also check from underneath, this can sometimes be patched up but if not this will be quite pricey as there is a fair bit of welding. prob only about £60 for panel but double that for fitting at least?
- rear arches - if you can lift carpet from inside this is better as underseal will hide the damage from the outside. again these can be patched more easily but still cost in labour (prob £20 an hour min)
- rear door arches - these are the bits you see when you open the rear doors. they are £48 a side and cannot easily be patched.
- sills - inner sills you need to get underneath and then look back out to see the back of the sills. these again can be patched. outer sills are hidden by plastic trim so look the best you can. these can be replaced by boxed section at £30 a side materials.
- front inner wings - dont be scared to poke underseal hides stuff, again you can patch these
- headlight bowls - open bonnet and look down behind headlights best you can - more difficult to patch and a pain to fit, they are about £75 each
- whilst bonnet is open check top of front arches, inner wings and battery tray less underseal here.
- floors - rear floor doesnt tend to rust but fronts will especially in front out corners and up behind pedals and same on passanger side - lift carpets if you can - if soundproofing is wet its probably rotten.
- front seatbelt mounts. unlikely to be able to check inside as difficult to get past interior, look underneath and poke around the bolt.
- alphine windows (the roof ones) they like to rust around there and can be a pain to repair, but can be patched.
- check body mounts between body and chassis, these are cheap to buy £10-15 but can be a pain to replace.
- check outriggers and rear crossmember that runs under boot floor. again parts are cheap but labour high.
- check for oil leaks, if its not leaking there is probably no oil in it
mechanicallycheck for smoke, check boxes and just general driving but as many have said before the greasy bits are easy to fix, the body is more important.
I am sure more will add and correct me, I hope I havent put you off too much.
to save, buy a welder (£120) get to a soft drinks dealer and get some co2 (£30) and learn to weld.
i bought a 2m X 1m sheet steel for £30 then an angle grinder with a wire wheel and lots of black paint and waxoil.
any further questions that I may have missed ask and I will answer the best I can, but i cannot guarantee that will be the best answer