Regardless of what happens, you've got to factor at a bare minimum 4 mags into the equation, you cannot run on a single mag, you'll be in games for maybe 2 minutes.
WE G36's are solid, you can usually get them with an NPAS and a few mags for well under £300 - the mags are really cheap as well compared to just about anything else. These are budget guns, so performance will be satisfactory, but you're not getting any form of laser beam out of the box.
Double Eagle N4's that are UK spec really need a lot of work to be usable - Minimum to work is replacement nozzle (usually guns modify), replacement hop bucking (most suggest Sniper Mechanics Flamingo, but they're fragile as fuck), Replacement inner barrel to run to the end of the muzzle - UK spec ones come with a 180mm barrel in a space big enough for a 250mm to fudge under UK limits, then an NPAS either Jaeger or Federal Concepts. By the time someone has done all of that, then decided to sell it on, you're basically paying £300 used with a single mag, and you can't run them on a single mag.
Do not buy Cyma Gas System - they keep getting labelled as MWS by retailers who don't give a shit, they're not, they're at most 30% MWS - where that becomes a problem, decent spares are non-existent and the ones that do exist are craptastic and overpriced.