I have an M50 in my e21, and it's... snug (can't get a finger between the firewall and the intake manifold and there's a max of about 40mm between the front of the motor and radiator, which is an SR20DET unit - and probably not up to snuff). I didn't do the swap, but purchased it "running" and have been cleaning up the po's mistakes ever since (credit where it's due - he got it in the car and running). I should point out that the front clip is still 100% intact in my car, although I'm considering cutting it out to make room for a bigger radiator and fan.
Other notables:
I have the ZF 5 speed from the e36 M3/e34 in mine - it's probably overkill and definitely a tight fit... either way, I'd try to start with an automatic car if you can since it gives you a fair amount of extra clearance and I think any of the boxes you're going to use are going to be a fair amount larger than the stock 4sp.
I highly recommend doing a complete fitment WITH SHIFTER prior to making any permanent engine/trans mounts. Also, and this may seem like common sense, but when you have everything where you like it, make sure the hood shuts... it's a tight fit vertically too. Anyway my shifter leans slightly towards the passenger's seat, so 5th feels a bit further than normal when in a proper racing harness. That said, you should be able to find stock BMW parts to make your shift linkage/platform... I think I used a combination of e30/e34 parts (don't hold me to that - I grabbed a bunch of crap out of a random parts box), which works pretty well (po cut the stock e21 plate, bolted it to the floor and cut/welded/bent a "custom" shift linkage - this did not work quite so well).
Also, you're going to have zero clearance for manifold/headers... I'm sure you can build some. In my case, po used the stock cast manifolds with a pretty janky collector... I rebuilt everything with two of the shorter M3/328 tubular manifolds (one is longer in stock form - I picked up two of the shorter versions from a local dismantler). If you take this route, you'll still have a bit of welding to do, as I'm pretty sure these only come in OBDII flavor (so you'll have some air pump lines and O2 bungs to weld shut since OBDI only uses one after the collector).
I'm sure there's other points I'm missing, but that should be a good start... feel free to message me if you like. Good luck and have fun!