OP: You list an MP5 in your sig line. What's wrong with those? Solid guns, mags fit into a variety of pouches, easily setup for in/out door play...mags are cheaper, easier to locate, parts/accessories common. That to me would be a more sensible choice than a p90.
Definitely look into KJW 1911 Hi-capa (who doesn't want more capacity?) CO2 pistols. Nice weight, great kick, and perfect for a secondary.