Expect all but a fully serviced machine to need bed feet and a belt. Make sure the bobbin case is there, it's one of the most expensive "parts" you'll have to buy, and watch for ads that say something like "Old Singer Sewing machine" or "Small Singer Sewing Machine" sometimes they even omit the "Singer" from the ad. Both of my sub $100 machines have been from ads that didn't say 221 or featherweight in them.
If you plan to carry it in a different case (like the soft ones discussed above) consider getting one with the case latches cut off. You don't need the key, and the latches won't matter. It seems to really lower the resale value so you can get them for a good deal.