Hi Stephen,
An '86 from the original owner? WOW!
Well, with only 70k it probably hasn't needed the valve seals and guides replaced yet, a typical concern on the early 300Es. This was updated on later models ('89+), minimizing premature wear of these items. Check and see whether the radiator has been replaced, and whether it has a metal, reinforcing insert in the 'neck'. These always fail, and could lead to expensive repairs, etc....
Definitely check the horrendous AC system for proper operation! (I expect this is the FIRST thing you check in TX anyway!) The alternator was upgraded from a 70A to an 80A unit in the '88 models, I believe - might want to check and see how it's holding up. Check that the seat springs are firm - they tend to go a little soft in the earlier models, and it could get uncomfortable. Having owned a 300E in the past will definitely help you notice anything untoward in the car's habits as you test-drive it. And finally, check the electric convenience items - seats, windows, sunroof, pwr antenna. All of the above is what I can muster up from personal experience with our 2 W124s - I'm sure you'll get some pro advice soon! Great cars, these - as you know. Good luck!
Regards,
-Nick