I didn't own a real DTM-car by the time I took on these races, so I used the Lexus Prototype-thingy (forgot the name, IS-F or something) which can participate in DTM-races for some odd reason. It's fast and makes an awesome noise!
Go for the Calibre or the Alfa 155, these cars have 4wd and are excellent cars for B spec.
You can easily win the DTM championship with them on A Spec and on B spec you can change the tyres to sports and then use them for so many different events, Bob is a different driver when he's in a 4wd race car, even with sport tyres on.
He basically rockets past everyone at the start, then cruises to a very very easy win.
I've got two DTM cars. The upgraded Mercedes 190 Evo II won A-spec very easily for me with soft racing tyres, a turbo and an oil change. Bob was a turtle in it though, so I bought a Calibra, slapped on a turbo and gave it an oil change. Bob destroyed the competition.
Conclusion is the Merc is fun, but the Calibra is a beast.