I'm going to swim against the stream here a little. It will probably be hard to find a cheap used machine that is in good working order, or at best it's a matter of luck if you don't know what you're looking for. There are some to be found, but it's hard to know without having it checked out by an experienced person. A bad machine, old or new, might sour you on sewing.
Whatever you buy, do your homework. There is a lot of information on this site about various kinds of machines, and you can also usually find a yahoo group for devotees of specific machines or brands and vintages. If you are considering something new, there are reviews on sites like Amazon. User feedback is not always reliable - sometimes the user is quick to blame a machine for something that resulted from user error, but it still might be an indication of how user-friendly a machine is.
If you have the wherewithal to afford a more expensive new machine, it should hold its value enough to offset much of the cost when sold in a few years if you decide you just don't like sewing after all. Good luck, and let us know what you decide!