With 20/20 hindsight if I'd spent double what I did on a donor I'd have been on the road months ago and it would ultimately have cost only a couple of hundred pounds more. But then I wouldn't have had all the 'fun' of repairing my rotten chassis.
I'd advise any new builders looking for a donor to do what you've done and get a really good car with an mot that someone else has spent their money on. It'll pay dividends in the end.