Personaly, I have found its more fun and exciteing if you build everything from scratch.
Buying kits are great for the few that maybe doesnt have time or the tools to get some of the things done. Such as maybe sweating copper for a manifold...but usally there is allways more than one way to do something such as a getting a mesh toilet hose and useing that as a manifold instead. Some things may be a little cheaper, Other a little more spendy.
I am rather a cheap bastard so seeing a $100.00 bill disapear has to be a really good freakin investment. I work hard for my $$$ and dont want to see any penny waisted.
Just my thoughts on it...In the long run you might save $50 bucks...but you are able to custumise and know your system inside and out..and to me being able to have things the way i want it, makes it worth the extra cash.