Single biggest thing: if the cylinder head hasn't yet been replaced, be VERY careful not to overheat the engine...the OM603 has an aluminum head, and the original head castings have a tendency to warp when overheated. IIRC the replacement heads have been modified so that warpage is less of an issue, but it's still something to keep in mind. I've never played with a 603, but IIRC the original heads had #14 as part of their casting number, and the redesigned heads have #17 as part of their casting number.
If memory serves, the later 126 has a dual-function sunroof...the rear can pop up for ventilation in addition to the whole thing sliding back. This makes the mechanism more complicated and it may be more difficult to troubleshoot. What exactly is it doing? There's a possibility that it just needs to be lubricated...if so, DO NOT just use standard grease. M-B has a special sunroof paste that won't gunk up or anything like that. Buying a can of it from the dealer's hideously expensive (over $100 last I checked), but there are places online selling film cannisters of the stuff for MUCH cheaper, and a little bit really goes a long way...IIRC one film cannister can re-grease multiple sunroofs.
You found an SDL in good shape...I'm jealous
I'd love to get my hands on a clean black-on-black SDL someday...
