I'd stay away from Turbo/Super chargers as they dont offer much value for the M50/M52 engines..
The M52B25 only has 187bhp as standard, so if yours has been RR'd at 218bhp then your doing very very well.
Even if you swapped the motor for a M52B28 (the engine from the 328), fit the inlet manifold from your current motor and a pair of Shrick cams then I reckon you wont be getting more than 230-235bhp.... (circa £1500 all in)
You could always get a S50B30 (the early E36 M3 non-vanos) engine which will be good for 286bhp if its healthy and has the tappets done correctly.... (circa £2000 w/out fitting)
OR. Go down the S50B32 (Evo 231bhp) route, but beware of vanos issues and the reportedly fragile 6 speed box (circa £3k)
Any M engine will be more expensive to run and service, but the noise and the performance are to die for!