I have seen a fair number of M52, S52US conversion. I hear it is pretty much an easy upgrade (for a seasoned professional), with only some special modifications. I haven't ever heard of an S52 upgrades, but that engine will NOT be cheap. Figure at least 15k for the engine and tranny...plus any install costs. I would peg that upgrade at at least 25k.

A more cost effective alternative is to find an S50US from a wrecked 95 M3 and install that and at that time or later you can easily add a turbo or SC to get more than 321hp.

I have seen used- low milage 321HP euro S52's including wiring for around 4K USD and another $1500 or so for the 6 speed. It's not uncommon for people to overpay certain shops (they'll remain nameless) that think they are the only ones who can get euro-only engine's/parts. I'm very tempted to do this conversion but I like the idea of keeping the original S14 as the powerplant the way BMW Motorsport intended.