question about GBBR to HPA conversion

Hello everyone.
My friend want's to sell me his 'King Arms Colt M4A1 GBBR' very cheap..
Here is a link: http://www.kingarms.com/productpage.asp?prodid=1889
So i thought about converting it to HPA.

My question is:
Can i just buy a tank, regulator, hose and a quick connector to screw into the mag.. And that's it?

Hopefully you guys can tell me more about this, because i don't know ANYTHING about HPA..

Please post some parts and tell me what i need to convert

Thank you guys!