What car to restore out of these three?
It all depends on the level of restoration you mean to accomplish. The BMW would be the most expensive, in my experience as parts for the BMW are the most expensive of any car I have worked on. The Mini I have no experience with. $3500 would probably get the Mustang driveable depending on it's current condition. If it needs an engine overhaul though that $3500 would disappear quickly. I just finished restoration of a '65 Mustang with my Grandson, last year. We spent over $20,000 onit and did all the work except the body and paint. Again, a lot depends on the starting condition and how deep into restoration you intend to go. We started with a car that hadn't been driven in 15 years (his fathers former car) and redid everything on the car.