The DTM series started in 1984. The cars were only slightly modified from their road-going counterparts when it started out. In fact, homologation cars had to be built and sold to consumers. The BMW M3 and the Mercedes 190 2.5-16 Evo were two of the best known. The real DTM versions of these cars sell for fairly large dollars - at least $150,000 and usually much more. These cars could easily be maintained and worked on by a privateer since they are very close to the street versions.
Around 1994-95 the rules changed and the cars became more radical. There was no longer a homologation requirement and all sorts of aero, electronic and mechanical complexity was added. The series died off in 1997.
It was resurrected in 2000 but was now the premier touring car class in Europe, not just a German series. The cars were (and are now) more complicated than F1 cars! I'm guessing it would take $1/2 million to even get started. Then, I doubt the car could be maintained without factory assistance. Much like buying a used F1 car, as someone previously pointed out.
Here's a cool video of an old M3 DTM car being run by a privateer: