Going to play it safe and say either CYMA AK or G&G CM16. G&Gs are great starter guns and decently cheap, and practically everywhere sells them. You also have many options with individual guns - from Firehawk (tiny but loud) to Raider (standard M4) to R8-L (basically a DMR).
CYMA's AKs are normally really good, I used to have their AK-74M. Their AKs normally range from £130 to about £200. Personally I'd recommend their AKS-74u, it's short and great for all environments as well as being full metal. Forcecore AKs (made by E&L) are also a pretty good option.
Hopefully this is helpful to you!
Theta