At what level should I begin to invest in rifle creation, as an Agent?
never.
to give you a better idea, all weapon construction kit in game are:
1- useless, better buy/mission for weapons, the constructable ones plain suck and if you for whatever reason REALLY want them, its cheaper to buy them in store than to built ( makes sense, doesnt it )
2 - bugged. according to some patch notes lately they have been fixed, i didnt try though, due to 1.
the few weapons you can actually improve ( not build ! ) are of no interest to an agent.