Ahoy and welcome. I'd avoid the sub-£100 CYMA CM5XX M4 series. The internals are fine, but the plastic on that range is very cheap and toyish, and that's the part that you can't upgrade.
The G36C would be my choice from those three, I rate them as a platform. I
think the CYMA G36s have better plastic than their cheapest M4s, but would welcome confirmation. The internals should be fine.
I'd echo that the battery and charger that come with it are technically functional, but weedy and unlikely to last a day. I'd get straight onto lithium batteries rather than spending another penny on nimh technology.
I'd suggest something like 2 of these or equivalent (although I'd measure the supplied battery and ensure that these will fit in the stock). Note that the CYMA comes with a mini (aka small) Tamiya connector, and that means you'll need airsoft specific batteries (RC vehicle Tamiya battery connectors have different polarity).
https://hobbyking.com/en_us/turnigy-nano-tech-1500mah-2s-20-40c-lipo-airsoft-pack.html
And a SkyRC S65 charger from wherever you can find it cheapest. This looks like a significant added cost, but it's
really not worth cheaping out on chargers given the potential for things to go catastrophically wrong.
Oh, if you're not in a hurry, and intending to buy from bbguns4less, then put your order together, leave it in your basket for 24 hours and you may get sent a 5% discount code.
Enjoy the time and money sink.