Things to consider when doing this:
Whether or not you want an OBDI or II motor, the OBDII will be a bit harder to install as your car was originally wired for an OBDI motor.
Also, If you can afford to just buy an M3, that would most likely cost around the same amount after motor, and install.
The downside to doing this swap is the cost, but also you have to remember (the more positive side) your cars insurance will remain the same, even though it is as fast as an M3, you will be paying for 325i insurance but have an M3 equivlent car.
The insurance benifit isnt one that ussualy appeals to adults, but even if your in your 20's the insurance will be significantly less than that of an M3.