Treatland, dos cycles, Denny, are all great, we are lucky to have such awesome vendors in the community.
It's best to think about it as a whole system, start with what you want it to do, and go from there. Since there are only so many parts, over the years people have kinda figured out what parts make a good system. If you put a huge pipe on a small kit it won't run right, so stick to something proven. Pipe and carb are just as important as the kit, sometimes more so.