Honestly I wouldn't bother with doing anything at all.
Just buy a gun and play with it out of the box. The KA M4 comes highly recommended. You may need to change piston o-ring eventually cause they can be a bit flakey, and the cut off lever because they tend to wear out faster than normal, but other than those two problems they may or may not show up and it'll usually last you long enough anyways.
Buy a charger and some batteries from Hobby King. Honestly I bought a charger, power supply and 4 batteries for under $75 shipped to my door (I got the Turnigy T20 Pro PSU, Turnigy Accucel charger, and 4x 7.4V lipos and these lipos are dirt cheap).