Honestly, I'd be scared. Correct me if I'm wrong guys, but aren't short trips around town hell on an injection pump and injectors with respect to varnish/crud/gum/**** buildup?

For the AC;
Does it blow at all? If you have the automatic climate control, try setting it to "cool" and then go open the hood. Look down at the AC compressor (it will be low, mounted on the drivers side of the engine below the power steering pump (or at least it is on my '84SD)) See if the little "triangle" of black metal is turning. If it's not, reach down there with a medium length wrench and tap it lightly. You could just have a weak magnet, loose belt, too much of an air gap on the pulley etc. Thats probably NOT the case, but it can't hurt to check, and it's really easy to do even if you've got no experiance on cars.

If you can get the car up to look at the undercarriage that will tell you a lot about it. Check all the rubber pieces and hoses to see if they're still supple. If they're bulged, cracked, tattered and old looking that should tell you it's been on a "defferred" maintenance program. Check to see if you have vacuum in the diesel tank. With the tank around half full, turn the car off and go to the fuel filler cap, open it. If you hear "whoosh" thats probably not good, and you might then want to check and see if the tank is dysfigured and crumply.
Also, make sure you start it once when it's sat overnight if you havent already. This will tell you a lot about the engine. Does the glow plug system work?

If you havent already gone there, go to www.dieselgiant.com and click on the Mercedes Maintenance Page. He has lots of pictorials on how to do really simple maintenance items on these cars. You wouldn't believe how fun they are to work on.

Oh, check the brake lines. If they're stainless steel thats great because it means they've been replaced. If rubber and old looking that means they probably havent been replaced.

One more thing then I swear I'm done. Check the oil cooler lines. They're half rubber half metal and go to the oil cooler (tall skinny thing that looks like a radiator and sits on the drivers side next to the actual radiator.) If these oil cooler lines are leaking they should also be replaced. If they fail while you're driving on the highway it won't take long for the engine to lose all of it's oil and then you have a roasty toasty OM617. Which is generally bad news.

Hope this helps a little. I'm absolutely crazy over my 300SD. I like it as much as my w114 coupe.