Get a M4 10.5" > - Lots of people have them, you can share mags and what not. A lot of parts available aswell.
Everyone I know that has a KA P90 has had their selector blow up, thus resulting in permanent full auto all the time! Un-educated on this subject, but could be an easy fix?
Boots are a necessity, pick up a decent used pair at the surplus, I got mine from Army Issue, water proof for $55 tax in.
If you get a Revision Desert Locust use "FANS" for 30% off. Coupons float around on google.
Knee pads are a must too, I us Arc'teryx ones. Can't go cheap on your knee's or eyes, Surgery would be more expensive
.
Good luck.