A few more comments:
With an alternator conversion you need only a 3-position light switch. (A 4-position switch works but is not required)
A 10SI alternator fits easily under the hood of an M.
If you can, get an alternator with the LOWEST output rating you can find - 37 or 42 amps is good. (Output amps is stamped on the case along with "12 V NEG" and the part number). Reason: Higher output alternators need higher RPM to turn on after startup.
Also us the SMALLEST pulley you can find on the alternator so it spins as fast as possible.