Any sewing machine (at least, any one I've ever seen or owned) could be converted for manual use, if need be, simply by attaching a foot treadle to the wheel on the side of the machine. So if this is just about 'what if someday I didn't have electricity' you don't really need an old nonelectric one per se.
Some people LOVE the old foot-treadle machines, though, and would not trade them for an electric model no matter what. I think if you do some googling you're likely to run into their websites and you could pick up some useful pointers about which makes/models are best and how to make sure one is in good shape. I see them for about $25-75 sometimes in thrift stores around here but I don't know the mechanical condition of those.
Good luck and have fun,
Pat
