depends on how much rust and how much ur payin for the car, might work out cheaper to buy a second hand shell with id and chuck everything into that, also when u come to sell it a lot of people will be put off a car which has had shit loads of welding done to it, if u got a garage to restore it ud proberbly be looking at a huge bill as preping/ removing parts and welding takes quite a bit of time most garages charge about 50 quid an hour for example to replace a rotten bulkhead (underneath the battery tray and is a dash out job) and a few holes on the floor normally takes a good 3 hours to properly and if the chasis rails need doing (most do) its a tank and rear wishbones out job which takes a couple of hours inc welding
if u do get it it would be best to strip out everything thats needs to be removed and do as much of the preping as poss and if u feel really brave get urself a welder and some scrap panels and practice like hell!!!!!

hope this helps sm