Well power can be got from the 2.5, but you would be best aksing on the E36coupe forum for that I guess.
I was hoping you were going to say the 2.8 because that can be easily tuned for good power, but with a bit more work it can be tuned for decent power
M3 3.0 S50 shouldnt be too hard a jump as most of the work would of been done fitting the M50. In saying that, there isnt that much work to actually get either engine in.
You would need to sort the exhaust manifolds, possibly a new exhaust and a bit of wiring (depending on the differences between M50/S50). The inlet plenum would need modding, because if I remember rightly, your car has the Porker 944 brake servo???
Not sure what gearbox youve got (E34 5 series rings a bell), but it should bolt up to the S50, whether it could cope is another matter.