The leak is probably the timing chain cover (I hope I got that term right). That's where mine leaks. It is a fairly simple repair, but I've not done it because it does not lose much oil, and oil is cheap.

Check to see that the valve stem seals have been done. They usually go out between 60K and 80K miles. With a car as old as the one you are looking at, they probably should be done if they have not already.

Air conditioning is another potential trouble area that is expensive to fix if it is important to you.

Check the rubber, from the suspension bits and pieces to the flex disk. There is lots of it, and rubber responds as much to age as mileage. Some are inexpensive and easy to fix, others are not (like the rear differential mounts).

Of course check the paint and interior condition (dash and leather cracking, for example). Those can very expensive to fix.

There's other bits and pieces, but a good PPI should find those.

And good luck! My '87 300E still runs and looks great.