Are you talking about the W123? These are great!!
Just don't pay too much.Also there is no 300TD
that you should avoid.Well now that I think about
it,do check the floor boards under the carpet for
rust,especially at the accelerator pedal and the
base of the pass/driver seat rears.Make sure it is
not 'soft',but thats about it!

here are some ideas:
1980 - no turbo, ~80HP, other years were turbo.
1985 - best year for rear end gearing for highway cruising.
1987 - motor change with hydraulic lifters, no more valve adjustment as
regular maintenance.

I'm considering a 300TD wagon and I'm wondering if there are any years to
avoid?

Thanks in advance