I know that you need a RP for all the stuff to do with cars that you mentioned (your DL and buying a car).

As for who to the buy the car from, I take it you mean buying a second hand in which case I wouldn't recommend used car dealers. The cars that they carry are normally painted all over the place, more expensive, and think about it this way; why would you put your car at a used car dealer instead of sell it yourself..because it's royally ****** :D. So what you'll have to do is take a look at the ads section of this website and also go to qatarsale.com and look for low mileage expat owned cars and never buy before you get the car inspected (preferably at the german specialists center).

Good luck!