A couple of links pulled together...obviously

For quite some time, I have been of the opinion that any of the early BMWs that are powered by m20 engines would make good candidates for the m or s/50/52/54 24v engines. As an owned of an e21(first generation 3-series), I've notices more and more of them being done and getting finished. A couple of posts down you can see that I featured an e21 M50 conversion that went to eBay.
Several of these builds have been documented on the forums and here are the links:
This is the build thread from the blue e21 that went to eBay. Very useful but long.
e21 m50 Build thread
Again this is a build thread of an e21 with a m54 this time.
e21 m54 Build Thread




Engine weight is always a consideration. With these vintage vehicles one has to be careful not to drastically disturb the balance(weight distribution) of the vehicle. These are all general numbers and are POORLY SOURCED at best. Feel free to leave comments if one of these needs contesting.

M10=
M20= 258 lbs
M30= 315 lbs
M50= 299 lbs
European M52=260 lbs(alloy block)I am under the impression that the alloy version of this engine can be sourced from a US Z3 2.8 m52 as well.
US Market M52= 308 lbs(iron blocks)
M54= not sure of the weight but aluminum block and head

So, for fantasy's sake, let consider an obd2 m52 sourced cheap in the United States. I've seen m50s with the correct oil pan(m50 e34 oil pan) for less than $300 but here is a "complete setup" for $1200. The gain is, while a much broader torque band and modern tuning capacity, only about 25hp over an m20b25(168hp vs 193hp). This is where I think the necessity to tune the m50/52(very much as the guys below with the e10)becomes apparent. In my opinion, the m52 above needs a small supercharger with only 6-8 lbs of boost and these cams and an obd1 conversion. Even without the supercharger tangent, that list would grow as you did the shakedown and management and etc but the end point is still the same. This a conversion that can put a vintage BMW daily driver's performance into the 300hp range without compromising drivability. Check out the piece below to see what can happen with infinite time and money.....

Just to leave a good taste in your mouth after all that "technical" reading, here is a 2002(e10) with an m54 twin-turbo swap.
E10 m54 Beauty
Really take a moment with that page. 330lbs of thrust in a 2500lb car sounds like fun to me.