I'd avoid the low end CYMA CM5xx M4s. While the internals are fine, the plastic feels (and is) really cheap. When I say I have no regrets about getting one, it's more because it was a decent base to play around with internally and externally, and it's still in the backup-backup bag as a last-resort gun.
As far as I'm aware, there isn't a bad budget G36. They all have a rotary hop unit, the length is fine for CQB or woodland, and I really like the ergonomics. The only downside is that the battery space isn't great, but it's not really a problem either: I run ~1400mAh block batteries and always change at lunch anyway. You can run G36 mags, or with a ~£10 magwell adaptor, it'll take M4 / STANAG mags instead.
I'm happy with my Jing Gong G36C, it's my my go-to now. Based on a disassembly video, the CYMA looks essentially identical, and CYMA V3 gearboxen (as used in their AKs) are well regarded. That said, PatrolBase have an "Evolution Airsoft" for £10 more which has a microswitch (meh) and a quick change spring (nice). That's more of an unknown brand though.
Might be worth a punt, you might want to stick with CYMA, but either way, I'd recommend it as a platform.