i'd say stock internals its best to use a ct9 hybrid although i think you can bolt a K26/24 up to stock manifold with an adaptor plate (someone correct me if im wrong!) Although its best to remove the stock manifold if your planning to use a bigger turbo as theres restrictions in runner 3 afaik which will lead to piston/ringland failure if too much boost is run, Best to go for an aftermarket mani from a trader on here before thinking about a bigger turbo! SPS do nice ones, not heard any complaints yet!
Tired so some stuff may be a bit off but im sure others will clarify/correct me!
Rich.