My four top gun regrets include not buying an M17S, I instead bought an AR-15, i was then 18 and it was one of my first gun purchases. The M17s was brand new for 599 with 2 30 round mags. The AR was 675.
As for the gas piston kits. I have read that they cause excessive wear to the front/bottom of the buffer tube because the bolt carrier is being pushed backward from a different point causing it to ride slightly canted. I'm not sure they are a solution.