Who says I know what I'm doing when it comes to sewing machine repairs?
Seriously, if you narrow it down to one brand (Singer), one type (round bobbin rather than shuttle bobbin), and one basic motor setup (belt drive), it isn't too hard. Scrounge up three, and two of them just need cleaned up and tuned. The third will do for parts, maybe. . . .
The cleaning is mostly just getting rid of the oil that has dried like lint-and-thread-reinforced lacquer. If you use the "normal" chemicals for something like this (brake cleaner, engine cleaner, acetone, etc), you'll damage any paint or stencils. Old-formula Break Free CLP works well, as do Kroil and Eezox, and they don't damage finishes. Once you've got things moving, you can figure out what you're missing for parts. If you pick the right machine, most of the small likely-to-be-AWOL parts are available. You may have to clean/adjust the tensioner--dental floss works. If there are problems with the motor or foot control, I just replace them (this is why I avoided machines with built-in motors).
After a half-pint of oil and a couple rolls of paper towels, you're ready to start tuning. This is mostly adjusting tension to get the stitches even and the tension (bobbin thread and spool thread) adjusted right. It is even less exciting that it sounds. Tweek the screw, sew a few inches, check it out, tweek the screw and try it again.
Over the summer I accumulated a Monkey-Wards free-arm plastic-fantastic, a couple of Singer 15s, and a post-war Japanese clone of the Singer 15. The Monkey-Wards took a few minutes to get working (helps if you thread them right). One of the Singer 15s had a cracked body and became a parts machine, the other and the clone took a while (and most of the small parts from my parts machine) to get running. In October I picked up a post-war Singer 99K. The 99K took a couple hours but came with all the attachments (don't know what I'll do with them, but I have 'em!).
I kept the Monkey-Wards machine for repairing and tailoring off-the-rack clothes, and the 99K for work. Total cost for all 5 machines, shipping, and misc. parts/supplies, was probably around $120.
On the good side, both machines I kept use the same needles. One the down side, I wound up with machines that use different bobbins (Type 15 and Type 66). Needles and bobbins are available as close as the nearest department store, as these are the most-common needles and bobbins around. The Monkey-Wards will probably last a few years of heavy use. If the 99K is kept lubed and cleaned it should last forever.
I'll probably pick up another 15 and convert it to a hand-crank, for occasional use in specialized circumstances like working with multiple layers of heavy fabric, etc.