Some of the ones that you mention can be great cars, if they were taken care of, but those won't be cheap (relative to their market segment), and they will have records...my 1994 850 was a great car. It was purchased from the original owner, who kept meticulous records, when it had nearly 200K on it, it ran flawlessly (it was killed by a Caddie in a collision...).
If the manual recommends premium, then I would use it. If you're using regular in a car that recommends premium, you're counting on the knock sensor to sense detonation and the ECU to pull back the timing a few degrees to prevent it...
I would rather not count on that...but that's just me...