Dude is right, start by reading the "how to build" threads and Newbie section on rccrawler. You will learn a ton.
From there you are only limited by your imagination and ability. I went with a chassis kit containing all the links, link mounts, steering setup, etc, because it was my first crawler, and I wasn't ready to fab everything myself (not that I couldn't, but I was lacking patience). It cost a bit more, but but I was really happy with the end result.
My other piece of advice would be to come out to a comp or GTG before you buy or build. There is a ton of info and pics online, but they really aren't a substitute for giving somebody's rig the once over with your own eyes and hands. It will help you get a better feel for what you want to fab and what to buy.
Todd
