from my experience, shoot for a HK11e. itll come together a little easier. And far as the quick change barrel goes, you are on your own. Hero's makes a drum and a c-mag for the G3. The c-mag is a bully but in real life you would see more 50rnd drum around. G3 c-mags are not mil-spec.
http://heroarms.com/hero%20products%203.htm
My c-mag cost 150 so check with hero's for a local dealer, i saw the drum overseas for $70us at cobrayairsoft. The CA kit IS heavy but wait until you are hold three more lbs on your gun because of that magazine. Mine is around 11lbs with the battery.
If you need a solution for the k grip go out and buy a rail and make it fit on your fore grip.They are $10-50us depending of the size. Mine is from NCSTAR and they may have a red dot scope for you if you check out their website. Ill send a pic of my gun tonight but here is some more real steel.
With the money you save on not buying the RIS/RAS (which I have yet to see on a HK) just to use a vertical grip you can go buy that drum mag or a some other accesories worth their weight in gold, like a good sling. THAT THING IS HEAVY! Your fore grip might wobble some like mine did, try to fill up the gaps with duck tape or black tape.
another option is to let the G3 experience some regression. this is my G3 before I got my c-mag. Alot of sawdust and time but its sexy and the wood stocks can be found for <40us.
sorry about all the big pictures
If you want to make an aeg a SAW i think their nothing to stop you except that you will have a SAW and the maintenance involved. I'm trying to prevent some of the damage by not increasing the the ROF or the FPS over 350. Just try to build your gun so the cheap thing break. For example, the piston instead of you mechbox. Good luck!