I eventually plan on swapping out my engine for.

I would steer you away from that if possible, too many things to go wrong in the swap and the E46 M3 motor will not accept an automatic transmission from the 325 due to the electronic incompatibility. I would consider enhancements to your present car rather than replacement of the engine, the M54 motor comes in several displacements and can be tuned to make excellent streetable power at a considerably lower out of pocket expense.