To get into it, you could pick up decent Deore level shifters and derailer for pretty cheap and they'll shift reliably and be tough enough for real trail riding. Ebay for cheap but great condition used would probably be your best bet if you wanna stay super cheap but decent parts. If you decide you really wanna get into it then you'll be dropping more money later on, but that's gonna happen either way so you may as well start out with cheap but decent until you get into it, learn a little more, and decide what you really want. I agree that just sticking the biggest front tire in there possible is a good bet to start out. Either way, keep the fork in case you decide to sell the frame someday. You'll want to have them together to sell. For now I say just pick yourself up the Deore stuff, some cheap cranks, handlebars, V-Brakes and levers(Perhaps all used from CL or something.) Add a cheap wheelset and start ripping up the trails. Go ahead and start saving now for when you decide you have to have a new bike. It probably won't take long.