Well if you've got to wait 4 months, that's roughly 4 visits, I doubt you'll be going more often than that because not even I do lol.
That's £160 over the next 4 months, which is the same as the cost of a gun.
But, if you buy the gun, you'll have to still pay to play at the site, as well as ammo, batteries and chargers and some spare magazines. Plus if you're using your own gun you won't be renting their overalls or face protection so you'll need to buy those as well.
So if you buy your own gun it's going to cost you miles more than if you rented for the next 4 months.
If you don't care about getting a two tone gun, or a realistically coloured one, then just get the two tone, but if you don't want to have a brightly coloured gun, buying one, if you're using it to get UKARA is completely pointless. Especially considering, as I've outlined above, how much more it would cost you if you bought a two tone now.
I'd say rent for 4 months, get UKARA sorted as soon as you've had your 18th birthday, then buy a proper gun, not a horrible bright green monstrosity lol, you'll save money that way and you won't need to buy another gun once you get your UKARA in order to make it worth having.