I just bought an 84 300D back in early Feb. I would say some things to look for on the body would be the rust underneath. What I would recommend is to give a gentle tug on the fuel/return/brake line grommet/brackets. What i found with mine is that the rust is not visible because the vehicles are so awesomely undercoated. However if the car has been up north, it is probably seeped through with salt and the crystals will rust out around the bolts where they go through the floorboards. ALso, look at the jack fittings for rust around them. Finally, on the subject of body rust, look on the bottom edges of the doors for rust bubbles that may not be visible on the outside view of the door and form at the seam etc.
As for the mechanical aspect, I am an moderate DIY'r, but I would look for vibration at idle on the hood. If it dances at idle, there's probably motor mount issues. Also look for play in the steering wheel. It's easy to adjust out if it is not already adjusted to the limit. Look at the boots on the rear axle joints. Are they cracked or open? What about the ball joints (upper and lower) on the steering knuckle? IF the boots are shot, they will need to be replaced which is not cheap if you have it done by a shop. Do the brakes have excessive shimmy in them when you stop from higher speeds (say 55 mph) in relatively short stopping distances? If so, the rotors probably need replacing as do the pads. I have done much work on my car since I bought it and it is a true labor of love. I really enjoy that old car... and i use it for work a great deal, having put over 5k miles on it in just 3 months.
Again, I am just a moderate DIY'r but this is what I learned on my sedan and much of it applies to the TD as well. The rust on the bottom of mine goes all the way through to the cabin insulation on the inside floorboard. I can patch it to make do, but I only paid $3m for my car and you are looking at significantly more for yours. It's worth checking it out and having a mechanic test at that amount!
Hope this helps in some way,