Building (or re-building) a bike all starts with a quality frame. A used bicycle frame is a great option for the budget no matter how you slice it. After the frame, you can add, remove, replace, and tweak forever and not be done with your work of art in progress. And that's the fun for the do it yourself bike builder. Enjoying a bike you put some sweat into back in the shop makes the trail, track, or road that much nicer to travel.
I've been riding and building my own bikes for as long as I can remember. Initially I could only afford the cheapest of parts and most of the time, the coolest stuff was the "junk" I found in the desert that people had thrown over the fence at the edge of our neighborhood growing up in Vegas. Ah the memories... Now I can save up and buy whatever I want, within reason for a middle class day job budget, but the coolest stuff is still sourced from unconventional places like the net, thrift stores, and some of the cooler bike shops around. I guess it's like when I used to mix up different lego sets to build bigger and better things. I never let the boundaries of one set limit my imagination.
Now it's the same with building bikes. I'll grab the best or most interesting brakes, shock, handlebars, etc for whatever project I'm working on, whether it be cruiser or bmx trickster, I don't pay any attention to original specs or anything close to them. Whatever works from the parts I have, that's how I do it. And it all starts with a quality used bike frame.
The frame really sets the tone for the end project in a lot of ways. It's like my daughter picking the primary piece of an outfit that can be accessorised on an infinite number of ways. Everything you add to the frame is like a puzzle piece with no right way to complete it. Doesn't get much better than that for my creative thinking.
Find a bunch of great bike frames, from mountain bike frames to road bike frames on bikeframer.com and return as often as you like to check out the new models.