I am by no means an expert, having been working on machines for only a few years, and only an occasional sewer. That being said, you would have a lot of versatility in terms of attachments and power options with machines with the motor boss/rail on the side under the hand wheel. When was this boss/rail added? I don't know but based on machines i"ve had I would say late 191Xs early 20s. As I understand it the boss/rail was added for a hand crank and then used later for motor bracket attachments.
The most common Singers I've seen are 66s and 15s. 66s have a drop in bobbin, from the top, and are rock solid, but the feed dogs don't drop. 15s have the bobbin that loads from underneath, which I don't like as much, but newer 15s have the ability to drop the feed dogs. I use a 15 has a hand crank for the small repairs I do. 15s are rock solid also. Both use any/most straight/low shank attachments, and Singer even made one for doing zig zag. You can find 66s and 15s in perfect shape or in junky shape. All this being said, I kind of wish I had one of my 201s back, it's the drop in bobbin and you can drop the feed dogs, I'm just not a big fan of the potted motors.
I would be concerned with buying that 115 because of parts availability.