#1 The XM203 madbull launcher would mount on the gun but would stick out as far or a little longer than the barrel.
#2 Num chuck batteries dont fit, you have to get a special "crane stock" battery that is designed for the stock. They do have 9.6v 1200mah ones though and even better ones too.
#3 They do make precision barrels for this gun and its internals will handle a m120 spring, but unless you have lots of experience doing upgrades, I would not reccomend doing it yourself. Just get it upgraded when you buy the gun.
The M15A4 CQB is an awesome gun, works well and shoots stock @ 350fps and with a 9.6v 3300mAh battery in the stock will shoot 900 rounds per minute. I really like the one my friend has, works well.