Don't get hung up on the "full metal" thing. The polymer bodied combat machines (not the plastic ones, the nylon reinforced polymer ones) are as strong as if not stronger than metal. Plus if cost is the issue bear in mind that the guts of the t4-18 is going to be basically the same as the combat machine. The cost difference between the guns is more than the cost of a tight bore barrel.
Don't try and scrimp on the protective gear though.
You are facing the same dilemma that most do when starting out. Fact is that airsoft is an expensive hobby. You could save a chunk by getting a cheaper gun though.
Snuck up on an enemy position, pulled out a pyro, struck it, threw it. It landed all off 18 inches away. Marshals saw the whole thing. I laughed, they laughed, the pyro went off, it was hiiiiiiilarious.
G&P make an enormous range of guns. Well...when I say "guns" I mean M4s. All very slightly different. But only slightly.
I would rather like their masterkey though.
I don't think you can get sepearte fittings for the G&P ones but you could probably pick up the Dboys/E&C 3in1 version new cheaper than your mates one anyway. They come will all three types of fitting (including RIS).
Ah, right! You'll be needing a speedloader then! http://www.ebay.co.uk/itm/AIRSOFT-BB-BBS-XL-470RDS-6MM-SPEED-LOADER-FAST-UK-DELIVERY-extra-large-mid-cap-/371070847315?pt=UK_SportingGoods_Hunting_ShootingSports_ET&hash=item56658bc953
Right, so if you just want it as a non firing prop, why do you need a magazine? The bottom part as you say completely covers the magazine anyway so you'll never see it. You can't fix the mag to the clip to be able to slap one in like the film either because the innards of the magwell don't match the magazine used in the film...
There is an airsoft Colonial Marines group that might be more able to help out though.. https://www.facebook.com/groups/426066200772765/?fref=ts
Again - Viper 3 point tactical sling. It comes with a loop that will go through the sling point on your stock and a hook at the other end for the front ring. It also has the adjustment loop and fastex clip mid length like your SA80 sling does which will allow you to carry it in multiple configurations. Plus, being Viper - it's dirt cheap.
Have a look at the Viper 3 point tactical sling. It has a similar system to the one you already have but it also has a hook that will work with your front sling hole.
Nothing off the shelf will fit perfectly. But if you look on the DIY thread you'll see how to mod the mask to fit better. Any decent set of goggles will work fine but you'll need to make the effort if you want to minimise fogging. Revision Desert Locust seems pretty popular, could also try Bolle X1000 or my personal preference ESS V12 Advancers.