The battery compartment is in the foregrip...and it's a tight fit. Mini batteries only (which kind of suck as you upgrade the power and the batteries get old). You can put a 7.4v 2xxx mAh 15-20c lipo in there...that should last for a good long time.
The G36C already has a bottom rail I think...you just buy the side rail kit (two rails total) and bolt them onto the sides of the foregrip. Simple as turning a screwdriver.
The flash hider comes off and you install a 14mm -ve (CCW, counter clockwise) suppressor. BUT...since the handguard opening is pretty tight around the barrel/flashhider, you might need to mod the handguard so the suppressor fits. or get an extension so that it clears.
Best of luck,