I picked up an ICS M4 last year have since upgraded to metal receiver (upper and lower) and RIS. it is a little tight for CQB
there are a few minor compatibility issues but overall the gun is easy to work on with the separate upper and lower mech box. if you switch between indoor and outdoor a lot you can have 2 upper mech boxes (takes about 1min to switch), you can also easily hopup/barrel swap to use a tight bore for out doors
the stock gears are decent, I would replace the piston (poly-carb but full metal slice), nozzle with an o-ring, (consider the piston head and cylinder head)
if you can afford it go metal up front, it's more expensive to do it later, ICS at least uses better parts for metal (primarily upper gear box) than they do for "dark tint"