Unhappy with deal. Want to cancel Purchase Agreement.

Don't hire an attorney yet. Sounds like you haven't even asked for the money back yet. Just go tell them you can't afford the interest rate and ask to cancel the deal and see what they say. Tell them that since the interest is so high on a used unit, that you want to wait a few months and come back and buy a new one. Hopefully that will motivate them to keep you happy. Nothing says you really have to do that.