What the fck is a D4817..?
Serious answer: I'd just keep saving up, as aforementioned they're generally not gonna be upgradable anyway. What's more, magazines might not be interchangeable? Extra batteries would probably be compatible (if they're the same mini Tamiya connectors) but they'd cost at least a third of the price of the gun itself, each.
Could hire three times then buy from a European retailer like TaiwanGun or Gunfire (once you have UKARA) because guns from there should be cheaper
For your UKARA defence, should be three times within a year, BUT not all in under 2 months. Preferably the same site (otherwise you'd need proof from each one? I don't think anybody does go to more than one their first 3 times, not worth the extra hassle)
Then you'll fill in a form or summut I can't remember exactly and wait for confirmation you're ont database.
Bear in mind the self check thingy website is shite from my expedience and what I've heard from others, and probably won't work (as in won't display your record if you search for it) Speaking of which I'm pretty sure I have a UKARA defence despite being 17 and I'm down as being 43 or something (due to some cock up with the guy who used to work at halo mill I guess)