Denham,
Having driven both mercs and Bmw's in the past, I say go with a bmw... There was a time when mercs were better... late 80's early 90's.
For 50-60k I wouldnt buy an M3, A standard BMW costs alot to keep on the road, a performance one getting close to 10 years old would be a nightmare..
My mum recently sold her 328i Covertable, 1996 model... Again nearly a ten year old car and the bills begin to pile up... Traded for a brand new 325ci (or 320?) and that is an awsome car, fully custom ordered and a sensational car...
I have a 318is 1997 in sydney that my dad uses, 5 speed manual, everything the M3 has appart from the engine, great little car for a run about...
Bottom Line?
BMW before Merc. Later model, even if its costs more on the purchase price, you will save in future repairs.