Ultimately a good project depends on what you want the car for.
Ke70's can be allot of fun mostly because how cheap they are to obtain and you can pull them apart and put back together and at the end of the day you haven't invested a whole lot of $. R31 , s13-15 , r32-34 etc are all good cars.
Work out what you want the car for, what skills you want to learn and then decide on what you want. i.e. if you want to learn a bit of panel beating, buy something smashed, mechanics buy one that isn't running, or if you just want to get in and drive buy something complete.
Look for people who abandon projects, and buy something that is half done mostly for the parts it comes with. Half finished projects are often abandoned either because of lost interest or a mistake made. If it is lost interest they are worthwhile.