Hi Malone,
Don't rush into buying one of those old clunkers, do some homework first. Also, get at least one lesson -- this will show you how to uphaul, steer, turn around, and set the kit up.
I learnt on something quite similar, they are very forgiving and easy to learn on. If you're after the cheapest way to get windsurfing then you've just found it.
However, make absolutely certain that what you buy contains all the parts needed to get windsurfing. This includes: board, mast, sail, boom, mast extension, unijoint, fin, and daggerboard (if necessary). If any of these parts are missing, then they will probably cost more than you paid for the rest of the kit, if you can get them at all!
Also bear in mind that after 6 months or so you will be looking to upgrade, and will have to buy all new kit. But $50 is a cheap investment to see if you like the sport or not