I am not a big fan of the new machines; like many others, I vastly prefer the older mechanical workhorses. Whenever I have a machine serviced or when I'm talking to an OSMG, I ask them what their favorite models are. The Kenmore 158 always comes up along with the Singer Slant-o-Matics (301, 401, 500). I love the vintage Vikings of the 6000 series. I hear the old Pfaff 130s are good and I recently scored a Pfaff 1222e for $200. If you want to do FMQ, look for a vertical bobbin. You won't need fancy embroidery stitches.
Where to find them? I would most certainly start at a dealership for a good brand and look at the machines they have taken in on trade. I got my wonderful Pfaff 1222e at a Bernina dealer. It came completely tuned up, with accessories, and included the dual feed system that does not require a walking foot. But you have to be firm and look only at their used machines. Perhaps someone else can check them out while you study. ;-) Craigslist can be good but you'll need to have a SMG lined up and if you do, you should enlist him to help you with your research and search. They often have trade-ins available for sale too. Items from CL often must be serviced and you may have to pay for shipping if it's far away. I'd still prefer a nearby dealer anyday.
However, before I do anything at all, I would find out what model Singer you have, come back here and tell us what's going wrong. There are MANY people here who can help you solve your problem(s), and the older Singers were made to last and were easy to maintain. As an alternative, you can always take it to a repair guy, but he'll charge you for the service, of course. But it will certainly be cheaper than buying another machine ... and probably faster too.
Just my 2¢.