first off - welcome to the site! you have big plans for that there RLS30
second off - there's a lot of questions in there! as far as the rear end, suspension, and brakes, there are currently a couple threads open about each of those topics. sounds like you're looking to DRIVE that car, so i'd recommend some big discs at all 4s, at LEAST an R200 (there is another thread right now that is discussing other rear ends for more durability), and suspension is going to be quite a beast to tackle. maybe some big sways, coilovers, etc.?