1stimer,
It sounds like the TD might be a money pit, so you are smart to stay away from it. The 1981 300SD, would be a W126. The 0-60 time should be around 13 seconds. It should be able to merge onto freeways just fine, with no sweat.

A W123 might be a wiser buy if you can find one, since you are new to diesels. They are much simpler in design than the W126, and resulting in cheaper repairs, as long as you do a lot of the repairs yourself, which in many cases are simple. A 300D, with a turbo, has the same engine as the 300SD, resulting in the same performance.

One thing to make sure is that the climate control is working proporly, and the rust is not bad, or that can be a big problem. The interior can be another spendy area to fix, if desired, so getting one with a nice interior would be another big plus. If you can get your hands on Illistrated Mercedes-Benz Buyer's Guide, by Frank Barrett, do it. This is a handy book that can help in the purchase of a Mercedes.