I've got the SRC G36 and it is pretty reliable for the price. I've had it for a few years actually. Internals are still all working fine so I'd say it has been worth the money.
Easily adjustable hop up.
But yeah it is a pretty good gun, quite a safe option for that price range.
Under £150 go for SRC or JG. In fact just go for JG, SRC are supposedly good but the JG are the ones used by a lot of sites for hire guns, which is an indication of reliability.
I have a Jg and they both have full metal gear box only the terrible guns on the bb gun websites Don't Have it. Try either patrol base, zeroone or proairsoft supplies.
The jg g36cs are beasts! My local site has used 3 for rentals for the last 3 years, they get used twice a month for full days and they are still fantastic! Build quality is pretty good for china made clones but you'd never tell if you didn't know