Another thing to look for is gas consumption. In good working order, the car will get (and is rated for) 17mpg. Mine's a 73 and I'm making that kind of milage on the highway now. A normally used/worn one will get 14 or 15 on the HW.
The best piece of advice I can give is to buy the best Mercedes your $$ will buy. It won't be for another bunch of decades that the 108s will become "valuable," so making back what you put into the car for repairs or restoration may be tough.
Mine's worth about $4k, but I've got $30k into it. Hobbies can be expensive if they need LOTS of TLC.
Parts are relatively easy to find. I go to the local parts places first, then online, then junk yards. I've never come up with a part I couldn't get.
Interiors are often cracked or faded. Finding a replacement seat cover set can go up in the thousands for new, hundreds for used. If you find used stuff, they can be recolored. I redid the interior on a 114 and it looked awesome, using a combo of dyes and new stuff. Carpet sets are also expensive new, but you can get the same stuff (perhaps different colors) from local carpet stores, and for much cheaper. You'll have to cut and paste the pieces yourself though.
Dash caps (not entire dashes) are available on eBay for $70 or so.
There are outfits that redo wood, but again, for a good chunk of cash. I redid mine myself after reading a few articles on stripping and refinishing wood. It still looks awesome and I saved tons of $$.
I vehemently disagree with any purist who states that a "non original" part, or something not bought from the Classic Center or dealership, shouldn't go into your car. My vote is that as long as YOU like the car, whatever it takes to restore/maintain the thing is up to you.
And when you come to the conclusion that you want a Chevy LS6 with an Edelbrock blower as your powertrain, come on back. We can help you there too
Or Gwunta can. He's doing it right now...