The stores out in the NT are supposed to be cheaper than closer to HK, so yeah, it's probably a good place. But a pain to transport if you live elsewhere. Remember you can't take a bike on the MTR or on a bus.
Or check this link for advice on a bike.
Beginners: Buy your first bike
Don't get too stuck on the idea of an XTC3- most budget hardtails really are much of a muchness, except for the paint scheme

They're okay for foothpaths, but if you really intend to get into trail riding, at least consider a budget suspension bike.
If you're looking for someone to ride with, join the Fat Monkey Yahoo group as they seem to be fairly active. The Flying Ball bike store also runs Sunday rides, so you could get in touch with them. Or it's pretty easy to hook up with riders going across on the Mui Wo and Lamma ferries.
Oh, there is also a guy who runs great bike skills courses where it would be easy to meet riding partners.
Cross Country Hong Kong