I don't even think you need a donor bike. That Super Sport looks like it will clean up nicely, While we all have our special sauce for cleaning up chrome, mine depends on the amount of rust. If it is light like on the Super Sport, I like to use Turtle Wax Chrome Cleaner.
It looks like you will end up with two bikes. Perhaps go over the Varsity first and then with that experience clean up and refresh the Super Sport.
Here is what my crank looked like before. I thought I would have to find something else because of the rust.
For the after, I don't have a close up of the crank and chainrings, however, as you can see below, it is much improved.
The Schwinn chrome was pretty good. The fact that there is rust means that cleaning it up only makes the rust less noticeable. Follow up with automotive wax to keep the rust from reappearing.
I have heard mixed reviews on the Huret/Schwinn Approved rear derailleur. I had the same derailleur on my mid '70's Astra and thought that it shifted well.
On the other hand, if you have a decent more modern derailleur, it should work fine with the Schwinn shifters. I have a SunTour ARX on the Continental shown above.
The front derailleur is a keeper in my opinion. Particularly on the Varsity since it has a slimmer seat tube than most bikes so why change a good thing.