parts for my '86 420SEL cost about $300 including cable. I recommend pricing them from Phil (this site) as I got a deal on OEM parts from him. If you're going into it plan to do it right and have years of trouble-free service. Fool around like the PO of my car did, and the sunroof can fail and ruin the cable quide, then you have more problems.
One thing about Mercedes-Benzes: I have never failed to find a part I needed, although it might seem expensive, at least it was available.