I wouldn't really invest much time or money into a CA sportline, by the time you're done upgrading you'll want to upgrade to a metal body... From there you would have spent more than if you would have just bought the full metal one in the beginning, then upgraded it.
I'd limit the upgrades to your sport line to;
Prometheus Neo Strike Hop up chamber (can be used if you buy a proline CA m4)
Prometheus 6.03 Tightbore Barrel 363mm length (Can be used if you buy a proline CA m4)
Quality hop-up bucking (Firefly, Prometheus soft, guarder clear)
Bushings, and shims for your gears.