Going to have to disagree with almost everything you just said, metal CYMA AKs are very affordable (under £200) very reliable and have fantastic performance out of the box, my mate has one that he got as his first gun over 3 years ago, still not done anything to it at all and it is phenomenal.
Ian_Gere on here will vouch for them too, as I imagine Finius will, also.
There isn't actually any need to upgrade them from my experience, and you'd have to be the hulk to break one over your knee.
In my experience SRC G36s don't really impress me. They're good for the price, but they're not anything like as good as all the other G36s, which are about the same price. My site use the UMAREX versions as hire guns and although they make great hire guns, I would never think of them as anything more than that.
Classic Army sports lines are the things you can snap over your knee. Every story I've ever heard about them has involved someone tripping over and snapping the stock and grip off.
Lastly, G&G Combat machine's are by far the cheapest choice, you can get a extremely high quality construction, excellent performing G&G nylon fibre M4 for a mere £100 brand new, everything else you've mentioned is £130 or more.
Where do you get your information from?