That really comes best by watching and studying the marketplace to get a "feel" for this car. Look up all the past recent sales you can find here, BAT, eBay etc. to gain some background to the market.
Buying in low with an eye toward flipping it later or 'getting my money back if I sell' is one path. Pick out what type repair/restoration work you DON'T want or can do yourself and find one that has that already covered. You will have to decide how modifications add or subtract value to YOU.
Good luck with your path!