You don't need to spend silly money to get a decent setup. You just need to be selective.
I wouldn't have the ARP9 as a starter gun. Not because it's bad. But because it isn't standard.
A regular M4 short variant with a standard crane stock will let you buy everything you need, then in the future you can buy a full length m4 and still continue using your mags and batteries without having to buy new again. It'll also mean you can get away with 1 set of ammo pouches keeping worn kit prices down.
If you went the ARP9 route you would need stick mag pouches, multiple small capacity batteries and a set of mags that only have a very limited number of guns you can use them in.
ARP9 mags are also expensive 300rnd £27. Especially when M4 standard 330rnd mags can be purchased for about £7 each.
Take for instance the short CM16.
https://gunfire.pl/product-eng-1152207749-CM16-SRS-Assault-Rifle-Replica.html
It has a higher base cost £191, but add 2 mags to that and it's suddenly cheaper. £205 Vs £234. That saving just gets bigger and bigger with every mag added.
It also has the ETU built in, so you get fet protection and 3rnd burst as standard.
There is also a kits available that lets you run pistol style mags in an M4. So it's not the end of the world if you still wanted that look in 12months time.
Component shop are cheap for batteries and chargers.
https://www.componentshop.co.uk/
For clothing go with military surplus.
DPM is cheap.
https://www.surplusandoutdoors.com/
Shirt/trousers/parka/PLCE webbing set/hat shouldn't cost more than £70-90
MTP can vary a fair bit but the base kit is normally cheap. The webbing prices can be extortion especially if you want a plate carrier.
Flecktarn is a great pick as well. The UK prices tend to be high but a few polish sites sell it at bargain basement prices.
I got for myself.
Shirt/trousers/parka/PLCE webbing set/extra pouches/and 2 hats for £70 delivered. (The postage was a full 1/3 of the price! they have some insane prices).
http://armyworld.pl/main-eng.html