See the thing about belts, they don't need to be changed until right before they break.
Just because the little book in your dash says 100k doesn't mean that's what you should do. The SVT is an interference motor and when the timing jumps you can pretty much kiss it goodbye. Take a look around and search for it here or even on google. Bad things happen when the belt breaks on an SVT motor.
To the OP, I'm a pretty technical and handy guy. If it weren't for so many focus friends I'd be doing most of my work all alone. That being said, I had OmniSpeedTuning in CA do mine this weekend and I didn't have to worry. If you're not experienced with it, I'd call around and see how much places locally would charge. Remember, phone calls are free.