It won't be a mistake if you look after it and use the right gas in the winter. For every 10 people that like and have no problem with a gun/type of weapon there are at least a couple who will regale you with horror stories that only they have experienced yet somehow are globally applicable.
The only thing I would add is don't buy a super cheap one (not that any GBBR is "super cheap" but you know what I mean).
Get one, enjoy it, maybe buy an aeg if you decide to start skirmishing.
I use Invader mags. They're pretty good for the money (box of 5 for £40). They work fine in my TM (ish) M4 and my G&G SCAR. They can be a bit rubbish at feeding the last few BBs though I've not had any real issues.
http://www.amazon.co.uk/Pro-Tec-M41-Protec-Holdall/dp/B004QMO7KC/ref=sr_1_6?ie=UTF8&qid=1403858510&sr=8-6&keywords=protec+holdall Protec M41 holdall (ignore the review on that link - I've had mine so full I can barely carry it and it's fine). Big enough to live in with plenty of extra pockets for all the usual bits and bobs. They also do one with a separate compartment for wet boots.
There's no need to be like that, the point is that if something is broken out of the box then it's up to the retailer to sort it out. While I appreciate that your time may not entirely be your own, surely it's better to get a warranty repair or replacement than potentially screw it up even more than it already is by attempting a DIY fix?
I think as per the above a lot of US sellers are using it as an excuse not to have to ship abroad/to the UK. I found a few guys on ebay that showed their listing as "may not ship to the UK" but on contacting them they were fine about it (Omega Mfg - they do a particularly nice FTS mount for magnifiers).
There's a few actually (mostly cheaper ones to be fair).
Even if you have to take the gearbox out it's still quicker and easier than splitting the gearbox which, for some at least is a step too far.
Interesting - I just clicked the link on the TR16 R4 commando and it shows as in stock.
Failing that I'd suggest the T4-18 or a SCAR L.
You could also try other stores (Zero One, Swindon Airsoft, JD Airsoft etc etc).
Green gas is a higher powered gas. You could try something like Abbey Predator gas or Abbey 144a. They are lower pressure gases that will lower your FPS.
Re-enactors are a different market though, they don't need functioning guns for a start.
The L1A1 is still listed on the King Arms website. Seriously - drop them an email and ask.
BadgerTac have a pre-owned FAL carbine if that's of any use? Might be able to get an afetrmarket barrel to get to the full length version? http://www.badgertac.com/products/airsoft-1/pre-owned/king-arms-fn-fal-tac-cabine-1-hi-cap-mag-pre-owned.html
The guy that runs it posts here fairly often. Very good little site with decent customer service! He even apologised to me for a delay on my order that I hadn't even noticed
I've been waiting for a Classic Army CA33E for well over a year.
Seriously - less popular guns are very unlikely to get production runs on a regular basis. You might actually be better off asking KA direct if they actually plan to ever make any more.
Personally - I wouldn't hold your breath. Not being an M4 seems to go against pretty much any production run these days. Better to scour the classifieds on here or Facebook and try to get a second hand one I would say.