I'm building a budget system with 2 12's in a ported box. I don't want to push more than 700 watts on my stock 80 12v alternator, so 700 watts RMS as far as I'm going for 2 ohms.
if you can find a zx750.1 used, that would be the ticket in my opinion. I think you can get a rockford fosgate Prime R1000-1 pretty cheap too and it probably won't strain the alty too much.