For regular maintenance, you should find a reliable shop that works on German and other foreign cars. There is nothing special about oil changes, except that it takes more oil than a "regular" car and the oil filter is not a standard spin-on type. I would expect that you could get oil changes at a non-dealer for arount $50-70 with premium, non-synthetic oil. Spark plugs are straight forward, non-platinum Bosch plugs. Probably an hour labor plus about $4-5 a plug. Brakes are also straight forward. Expect about 1.5 hours labor plus pads for front or rear. Pads and pins/clips could cost $80-120 (front or rear) at a shop. The key is to find an honest, reliable shop. I would avoid franchise places, and the Mercedes dealer will charge high, high prices, even for regular maintenance. Get the selling dealer to do as much as the initial tune-up and fluid changes as possible.

Maybe some of the forum members can recommend So Cal shops in your area for follow on maintenance. Good Luck!