I have found that "one-make" garages are usually the best to deal with. If you have an older car, find one that has "seasoned" mechanics that have experience working on those older models.
With our 'ol 88 Mazda 626, one of the local dealers has a bunch of mechanics with tons of "old" Mazda experience. Heck, they even keep good used parts around for people with older cars. They are one reason our 'ol gal still runs like a top after 15 years.
With a Mercedes, it's important to find someone like, well, Steve, that knows these cars inside and out. It might be an MB dealer, it might be an indie shop. Things I look for:
1. Knowledge and experience.
2. Tools. They need good diagnostic tools, and these cost money. I don't mind paying a higher labour rate to cover these costs, as it saves us all time and money in the long run.
3. Attitude. I only give my money to people that actually want it. Treat me with respect, and I'll give back the same. If you don't, I walk.
4. Shop organization. A cluttered, filthy, messy, unorganized shop is a red flag for me. Attention to detail is important.
Things I give to them:
1. Notice when I can. I don't rush in on a Monday morning and expect something non-critical to be looked at without an appointment.
2. Listening. I listen carefully to what the mechanic is telling me, so that I can provide the right information back.
3. Notes about problems. When it did it, what it did, what the speed, road conditions, etc were like.
4. Pay when the job is done, and don't gripe about the price on a job well done.
Excellent mechanics and shops are out there. It just takes time to find one. And when you do, help them to stay in business, as we all know what a treasure they can be.
1998 C230 "Black Betty"