Factory regulator is PLENTY good. I'd just advise only making adjustments to it in SMALL increments (then dry fire) only moving the pressure up. If you need to adjust pressure down, it's advisable to de-gas the gun. (air tank should be removable while still full, making it easy to de-gas the gun, double check on that) Preferably you adjust the regulator as little as possible. Find the sweet spot, and leave it.
I use a yong heng to fill a 9 litre tank. Then fill my guns from that. Your gun has a relatively small reservoir, so would not be nearly as much of a pain to hand pump as my 480 or 580cc bottles would be.
I'd pick up a decent scope, mounts, a moderator, and bipod if you don't have one. And plenty of JSB or FX pellets