Sent you a PM on the welding stuff -
As far as making it legal - unless you go the kit car route - I would highly recommend you buying either a wrecked car - or a very cheap car and going that route.
It's one thing to title the car - it's a whole other ball game when it comes to insuring. Having a valid VIN # is a great first step.
I looked at possibly doing this before I got the Porsche. BMW's are still a premium over a domestic mustang -
If I wanted the BMW - I would go with the 318i shell, strip it, then stuff a big block motor in it and have a great sleeper. Should be able to go real light with the body, especially if you d a majority in fiberglass. You better have madd welding skilz - cause the fabrication is not for the faint of heart.
My next car I will probably do an early Porsche 911 - 1970-1973 - stick on some wide ass flares and a later flat 6 from a 90's 911. The early ones are easy to keep under 2000 lbs - the later flat 6's of the 90's cars get to 300 hp pretty easy.