Welcome @Raddy
Of course this question has been asked a few times already so the first thing is to do some searches and read as much as you can.
As our new member Dan said, next would be to play the three games so you can be registered in the UKARA database. This has multiple benefits. The obvious one being that you can then buy a realistic looking thing (RIF)😁. (Otherwise it has to be bright red, blue etc.)
Another benefit is that you'll get some experience on how you like to play and what style you'll like. You already have a preference for an M4 but you can get long DMR type or short and light for sneaky attacks etc.
I also have a SpecnaArms (SA-E03) which I'm very happy with performance wise. (It could benefit from upgrades of course but plenty to have fun out of the box)
Other manufacturers to look at in that budget would be JG, CYMA, Double Eagle
Remember to budget for:
-Extra batteries (x2 Li-Po 7.4v 1400mAh or similar) (Deans is a better plug type if you can do some soldering)
-Intelligent charger (SkyRC S65 is popular recommendation)
-Boots with ankle support (you may already have)
-Eye protection (search and read for info)
-and other bits like carrier kit and BB's etc.