Hello Joe,
Welcome to the forum. It has been incredibly useful for me at the time of my research prior to buying my SL and it continues to be as I tinker around with it.
I bought my car almost one year ago and have seen and driven no less than 30 examples before I made my decision. Here's my POV on the red car you brought up.
I think the biggest plus about this car is the first owner... this gives you a car with a full history (hopefully one that tells about pampered treatment, full maintenance records and capable mechanics) and something to work with in order to determine the real shape the car is in.
Unfortunately, there is one common factor to absolutely ALL W113 SL's... that is: RUST. It's not a matter of if it has rust, but rather where is it. From the looks of the pictures posted, I'd be suspicious of at least a bit of rust on the footwells and trunk area to begin with... check also the rear wheel arches and the inside of the front quarter panels. The paint job looks cheap and quick, which makes you wonder why someone who kept the car in such good shape and for such a long time would allow it to be worked on by a sub-par mechanic or body shop. The answer is usually along the lines of "I'm ready to part with the car, so I really didn't care all that much".
The interior looks nice and I would say that for the year the car is and the color combination, you are looking at a very nice example. If the engine runs OK aside from the roughness at low engine speeds, you should still consider a significant amount to get this job sorted out (I would suggest you agree with the seller that he gets this done before you purchase the car - but be careful of what I mentioned before about people ready to let go of their toys...
All in all, and based on the description you gave us and the pictures provided and assuming there is only partial and repairable rust on the car, I am willing to say the car should fall in the $18K-$23K range. Definitely, 35 is high even for a car with no rust, mainly due to the poor paint job and the alterations to the car's original details.
Good luck and keep hunting
Andres G