I've fixed up a lot of old machines found at garage sales and thrift shops for a local non-profit.
I usually don't want to fool with them unless I can plug the machine in and make sure at least that the needle goes up and down. If possible, I also whip out thread and fabric to see if it actually makes a stitch. Also I look to see if the wiring is okay.
For $5, a machine can be worth it just for fooling around to see how everything works. Most of them, if they meet the above criteria, can be fixed up to work for somebody.
The model usually doesn't matter when they are vintage machines. Most of them are very similar in how they are threaded, how they need to be oiled, etc.