I've had a hand in building a couple cars in the past....

1962 Impala SS, had low miles and all original inside and out....the work to be done was minimal, taking the body down to metal, cleaning out any bondo etc....interior was minor seat work. Engine was a bottom to top clean up and replace. The money came in with the suspension.....Paid so little for it I could afford to outsource the interior work, engine work. Figure on doing a total rubber replacement, and dropping the gas tank. which will entail a lot of plumbing replacement, brakes will be a consideration, going back standard or adding discs....

The other was a 1972 Corvette, I could not do any of the body work due to it being glass, the interior was basically snap in snap out with door panels. The top was expensive (convertible) it came with the optional Hard top, a big money piece, 1 reason I bought it. brake work, engine clean up, the cars are notorious for bad u-joints and the rear suspensions are always a pain....It paid off with the top down driving, nothing touches that save for a 2 wheeler, but much safer of course.

Here in my opinion are the things you need to weigh in on when doing a hobby like this....

1- it is supposed to be a fun project, not a headache waiting to happen.
2-don't set a timetable on finishing it.
3-what looks good at 1st will always actually need work.
4-don't go into dream mode right off....keep a tight budget as best as possible
5-most important, don't rely on anyone except a pro mechanic before you put down any thought of getting into it, no matter how close a friend they may be.
6-maybe second most in importance....is this something you are doing for yourself or your friend and where he is at this point in his life.
7-heres the real question for yourself.....how much do you need to be talked into doing it....if it's always been a dream now is the time to put it in the bucket....Take your time, help out where possible.
8-if you go ahead with it, keep meticulous records with pics and all papers, all costs, all receipts..the more before/after pics the better....if you keep it in the family or sell it that info will make you better either way you go.

Another big issue is, how are you intending to use it when done, if it's an every now and then ride or if it's a driver to enjoy. If getting in to the drive it mode you need to consider putting in modern components IMO, upgraded engine etc....(more money). If it's a every now and then deal you could look more at doing a spruce up and stay as original as possible.