no one is gonna say you CAN'T do it, but its all in the amount of time you want to have it done, buying them is instant gratification, doing hours upon hours of research to spend hours building, and then testing can prove frustrating but be more rewarding in the end. however if you add up the parts to build them (using good parts) you're going to notice that you're not gonna be saving as much as you may think.
that being said i'm looking into lobbing the front of a quad off to start a complete front suspension from scratch