17th Sep is the third Saturday of the month, and the mage chronicle Nexus runs games on those days. Can someone from that campaign confirm whether there will be a game on that day and if any of the players would be conflicted if another game ran?
There are no conflicts on 22 Oct that I can see, although some people may be away for the long weekend.
Personally, I don’t think you’ll have any trouble getting 18 people on either of those days. You’ll have people lining up for another western larp.
Would you like financial help from NZLARPS? You could run the game as a project of the Auckland branch of NZLARPS. Put together a budget that will at least break even. The income from game fees from 18 players should exceed the expected costs of venue hire, prop and material purchases, plus any unexpected costs. If you want a spreadsheet to do the budget in, there is an official one here. The Auckland branch can fund the up-front costs. Note that for projects of the society, any profits and any equipment purchased for the project go back to the society, to help future larps. There is more info on projects here.