As a head's up, our weapon rules are reasonably strict on latex weapons. I can't say one way or another from just a picture on a website, but I wouldn't be surprised if that warhammer failed to pass an Alliance weapon inspection. The good news, though, is that monster camp (definitely at HQ, probably at every game) will be happy to lend you a replacement if it fails and plenty of players will direct you / help you get a weapon that passes in the future.
As to the rest of your request, anything in-period is good. I particularly recommend a costume with head covering (possibly optional) to help avoid bad sunburns and gloves (temperatures can get frigid at night). In terms of armor, you can get respectable armor value from a simple studded leather jacket or vest, which should be rather cheap to purchase or make. Also leather arm and leg guards are usually available at a discount at Ren Faires and conventions.
Most everyone will suggest that you NPC your first couple games, and I generally agree, but if you really want to jump into the deep end for your first game, feel free. Whichever choice you make, you should definitely read the rulebook pretty thoroughly first, including making sure you understand spell rules (even if you don't intend to cast, you need to be familiar with the rules if spells are cast on you). That should get you going pretty well.
-MS
