As a person who is constantly making costumes I say it depends entirely on the exact model of your Singer whether to pump any $ into it or not. Some Singers were great, others not so. I think $100 for a simple new machine is going to be perfectly adequate for your stated needs. So spending $80 to fix an oldie that may need constant care just makes no sense to me (unless it's one of those awesome REALLY old Singers). And don't spend $300 for the amount of sewing that you say you do. There have been good machines mentioned, but they seem like overkill for what you say you want to do.
Of course spend away if you see one that you just take a shine to...
