Mercedes StarTech just south of Downtown is a premier shop in this area - bring your checkbook. A Mercedes of Westminster could do a decent job on pre-purchase inspection.
One option is to gather some info and photos on the car and turn to the experts on the MB Forum like the one here:
I've loved these cars for years now... I've owned two of them ('85 300D Turbo and '87 300SDL turbo) - both were wonderful cars - sad I sold them. If you're looking at that '83 wagon in Ft. Collins - it sure looks like one heck of a car.
Anyway - I'm not an expert, but if you want me to go look at it with ya I can do that one afternoon. I'm in Westminster (120th and Federal).
Here is what I would be checking for:
1) Insist on engine stone cold before you visit so you can expeience cold start yourself
2) Check for excessive smoke at startup or normal accelertion
3) Check for blowby when running
4) Check for any sign of rust (they are notorious for rust spots)
5) Check the flexplate on the driveshaft (visible under the car)
6) Check brake lines, rubber hoses, brake pads
7) Check vaccuum operation (doors, trunk, trans)
8) Check for any hesitation on D, N, R, N, D, N, R and flare during acceleration
9) Check how the turbo spools up
10) If you are serious, its good to get compression test done on engine
11) Rubber seals and boots - check for cracks
12) Check for oil in the engine coolant
13) ALDA (anaroid valve) tweaked? - if so, ask why
14) Sunroof leaks - I drive through a carwash to make sure
15) Watch oil pressure - check oil cooler lines
16) Watch engine temp - these cars like to overheat and when they do they crack their heads
17) Make sure heater works (I don't like A/C myself, but it may be important to you)
18) Engine shaking at idle (mounts, engine shocks)
19) Ask for maintenance records (if any) - good owner will keep notes, receipts, notebook of things done
20) Check visual filters and fluids - dirty, clean? Will tell you a bit about the Previous owner's maint. habits
Forum I listed above will give you access a lot more info if you ask...
Good luck -