I've taken several cars to Mercedes Benz of Lindon for alignment. The service and parts department have treated me very well, done a great job and the prices are quite reasonable. Several of the mechanics drive 123's. Be prepared for them to tell you your car needs $1,200 worth of front end work (because it probably does) but just tell them you'd like the best alignment possible with the condition it's in. Should cost about 80 bucks.
Personally, I'd do the mechanics a favor by going to a carwash to steam clean the undercarraige before taking it in. I do the same before I work on the car myself and I like to give the technician every advantage to do a good job. They don't like standing under a grease pit any more than you. They will wash the rest of the car before giving it back to you.

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'79 300D, Ivory "Gunther" ......going
'81 300SD, Metallic Blue Silver, The Grey Ghost.....going
'87 300D Turbo ...gone
'05 CDI.... gone