I think its a great idea to build from scratch, I am currently doing the same using a spare CEN fun Factor chassis, Savage arms, buggy diffs etc. My only advise though would be to get a truck first and enjoy running that while you are building your 'custom' as a side project. There is so much you need but dont realise when you begin. Boring stuff like screw sets, brake systems, electronics housing, it could end up taking you years!!
If you are determined to start in rc this way though, I would start with something like a buggy chassis because they are very tough and have a host of compatible/ interchangable parts between brands. Hope this helps.