Although I'm in danger of being against the grain in this thread full of people in Fiestas sneering at BMW's, I thought it might be prudent to actually answer the OP's question.
Running costs are high but probably not much more than you currently pay for your DC5.
Servicing: Is roughly every 15k and varies between £150 oil services, £250 Inspection 1's and £350 Inspection II's. You also need a brake fluid change every 2 years and coolant every 4 years.
Tyres: Are between £400-£600 a set and last 15k on the back
Reliability: Fundamentally good but you will find yourself spending £100 here £100 there on minor things like sensors etc. Cooling system is weak - budget for a new radiator at some point on 6+ year old car with ancilleries @ £400ish all in. Only other main issue with these is suspension bushes as per any car, looking at about £150 a side for front upper/lower control arms.
Performance: Excellent - 230bhp, under 1500kg with a driver only means 0-60 in a shade over 6 seconds and an electronically limit top speed of 155mph. Obviously boring compared to whatever exotic the wise guys in this thread have, but still not bad going.
Often owned by: Arrogant morons who drive like idiots and think they are better than everyone else. Image problems aside this means there are a lot of cheap, ragged ones on the market that have been driven hard with little mechanical sympathy. Try and find a 1 owner car and don't buy from the bottom end of the market.
The models to get: The 330Ci Sport is the best model - M3 'inspired' bodykit (Unlike the 5 Series the bodykit is not identical to that of the M car), sports seats, M-tech suspension etc. These came in 2001 on a Y plate - before this there was only one model, the straight 330Ci.
How much to spend: About £10kish. Cheaper ones are shonky or low on kit unless you are happy with the 2000 models which are not Sport models.
