Regarding structural soundness, I would focus on two things:
Check the points where all seat belts attach for any rust of significance.
Check where the front seats attach to the body for rust as well.

That rust is bad, but it isn't going to break in half as you drive down the road. You just want to be sure it is safe in an accident. Clearly its safety is a little compromised, but you wouldn't be picking that car if ultimate safety was your only focus. I think if the rust at the seat and belt locations is fixed (or not present to begin with), then the car is at least usable.

Yes, there are a few w115s which go for more than $1500, but your car will have major rust patches all over and be in a terrible need of a paint job. It will be hard to sell a patchy looking 43 year old 220d for much more than $1500. Of course, if you get it painted professionally and totally fixed up, you could have a $5K car there. But you will have spent $8K in the process.

Enjoy the car for what it is. Practice some welding. Practice using POR-15. Etc. But don't spend $1K in body panels or $3K on a paint job expecting to get much or any of that money back. The market for a 220D is not great, sadly. So spend whatever you can on the car if you love the car. But don't try to do it for re-sale value.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).