Perhaps late to this party, but here's the link to the other pics and my thoughts.
Donor car decision..
IMO, to fix the gold car's taillight will require pulling to get that outward buckle removed. BUT - I don't think that taillight area will ever be usable - so you'd need to splice in a replacement piece regardless.
Based on what you described of both cars, I would recommend repairing the rust on the blue one.
Some folks have commented that there will be rust all over, and that rust never sleeps. I agree and disagree with that.
Only you can inspect the blue one for other rust - if it is truly confined to the quarterpanels - use that car.
If repairing the rust on blue, if you cut it all out, and weld in good replacement metal, you should not have problems again. The key is NOT cutting corners, and putting protective coating back on where prudent. You don't have to be fanatical about coating everything - once metal gets surface rust, it stabilizes, and you're not going to daily-drive in the snow and rain. So the rust problems you are dealing with is not going to recur in the same fashion.
I have done 2 cars of rust repair, with not-ideal tools. Cutting metal with a sabresaw and forming over woodworked 2x4s is no fun. But it can be done. And it makes a solid repair.
As for the engine - if you can get it running in blue, that's a labor savings. If it must be pulled, then the gold car is still not an advantage, it is only neutral (it's a pull and reinstall either way).
A complete/running/driving car is always less likely to turn into an abandoned project (as others have said).