I agree with the other guys that if you are going to do the swap, it seems to make sense to get the best engine you can for the money...all depends on your budget. I think J!m has pointed out before on this board that "speed is just a matter of money...how fast do you want to go?"
If your stock 4-cylinder is pretty low miles (<75K mi) and in good shape, you can get "M3-like" power without the extra expense up front by putting in a supercharger. I paid ~$3000 for the kit from Downing Atlanta, and another $700 for labor to install it on my 75K mi engine. I've had it in for ~10K miles, including a couple of autocrosses and small track days with no problems at all. The stock clutch becomes a weak point with this conversion, so I'll need a clutch soon, but on a car that's been driven hard for 85K miles, that's to be expected.
Just another alternative for those of us who don't want to spend all the bucks for a full conversion up front...
- DASC ~$3700 incl labor
- M3 Clutch 10-15K later at ~$600 incl labor (+$400 for lightweight flywheel if you so desire
)
- 325/328 brakes ~$800 incl labor when your stock brakes need replacement anyway
- E30 Differential ~$300 plus labor when you get around to it or yours starts to whine under the strain of you newfound power