There are some great trails at Glentress near Peebles. I've been over there a few times.
They hire great quality bikes out for 20 quid a day, including helmet.
They also sell these bikes, fully serviced, for £450 after they done a certain amount of hires. I can't remember the exact amount of hires they do before they sell them, but it was a surprisingly low amount.
Well worth hiring one for the day to see what you think and have a chat with the guys.