Always, always, always get the opinion of a professional mechanic in this situation. Neither you nor the lady are "detatched" from this decision to make it based on facts.
The big worries for this class and age of benz are structural rust and engine quality. A replacement tranny might only be 1,000$. That's not much compared to having to rebuild the engine because it's not quite up to par.
This is the right time of year to make a casual check of the engine. With the motor stone cold (sitting for more than 4 hours), start the car (follow the instructions about the glow plugs and all that). If you turn the key and it starts in under 3 seconds, the motor is most probably fine. The longer it takes, the worse off it is. This check doesn't replace getting a deisel mechanic's opinion, but it's a good rule of thumb.
As for the body, the rust you don't see is what you worry about. This car is a unibody, so there is no separate frame. Structural rust will kill the car before the paint starts to peel on the hood & roof. The area under the doors is called the rocker panel (or frame rails). Look at them (each side) carefully from 4 different vantage points. Usually, you'll find it cosmetically pretty on the outside, painted portion directly under the doors. But, that is just paint. Now, look at the very front (and rear) of this long, round, hollow rail. The front (and rear) should be sealed shut and you'll find a thick welded rail down the bottom (like a seam of a lady's stocking) from one end to the other. No seam means body work has been done. That means there was rust damage that had been filled in. The last perspective is from directly underneight. That seam is also the seam for the floor. See any along the way? There is suposed to be a thick rubbery undercoat. It protects the metal, but it traps water. That water will rust out the car. Now that you've been on your back peering under the car, check out the fire wall, both in the wheel well area and under the hood. The open area of the fender wells is cut off at the firewall with a bolt-on panel. Is the panel there? Is it bolted on, riveted on or held on with bondo? If they ain't bolts, then something has been done to fix a problem.
Finally, use your eyes over the rest of the car. Is it shabby? You might have noticed that by now. Wear and tear on the instruments, seats, door skins, headliner or exterior paint? These things are expensive to make perfect. Assume that whatever state they are in now, they'll stay that way. Can you stomach it?
Finally, if you've actually done all this in the presence of the car, you're probably hooked and you're going to buy it one way or the other. At least it's no longer a pig in a poke and you've had your eyes opened a bit. Use this information to come up with a fair price.