Sills, rear arches, battery tray and chassis legs. Condition of these for me is most important, other engine parts are bolt on, and whilst can be difficult to find, they're fairly cheap when you do.
Proper repair work to the body is costly, and given the age of cars a quick fix & bodge 15 years ago, will more than likely require a full on multiple panel replacement and respray - equal or exceeding the value of the car in most cases.
If it's an honest car which just needs TLC, then start the bidding at 3k, but be prepared to take something around £2600-£2700 for it.
There are very, very few cars which would command near £5k - G40s are often worth less than the sum of their parts!