I'm thinking of buying a used 2002-2004 BMW M3. What are things I need to know before buying one? Such as excessive expense..etc.. Thanks

You might want to investigate the possibility of an Extended Warranty Service Contract. Factory warranty in the US used to be 4 years or 50K miles whichever comes first at no cost to the owner. Thereafter, you can buy extended warranties from a BMW dealer valid up to 75,000 miles or more. There are also warranties available from sources other than BMW. Actually, I believe that the M-3s are so well built, you should expect no major problems if you keep up the BMW Service Contracts I and II through 100,000 miles. Ask around, you may find some low mileage cars around that have been well cared for and don't need any repairs.

I have worked in multiple shops and am currently at a parts store, so I can definitely attest to the cost of ownership on both fronts. The M3 has VERY expensive tires. A new set will cost you AT LEAST $800 for a decent type and a lot of shops won't even touch it for tires because it has mag wheels. Another thing is that the oil filter costs $9+, the battery (in the trunk) is $140, it takes a crap load of oil, they are riddled with electrical issues, it takes special fluids, and when your parts go out the labor will be high on top of costly parts. If you want a car with a lower cost to maintain then you can look at comparable Infinitis and Acuras.

Well get ready to hear the insurance prices on how high they will be. Also BMW M3 drivers have no friends on the road (meaning you will always have someone challenging you to race). You will need to take great care of this car and have a lot of money for gas.