What you can expect really depends on the kind of LARPing. Some are just small battles, some are big open worlds where you can do whatever you want and some are faction based, so much that you can't do anything if you aren't in a legion. But there are more different kind of LARPing than that.
As for what you should buy: Not cheap. It isn't cheap to start off with, but the things that are cheap are very poor quality, so stay away from that. The equipment does really depend on what you want to play though. Keep in mind that you should think like video game-classes except, the only limit is your own physical ability.
I plan to start LARPing this summer, but I can't afford full armour to start off with, so I'll buy a little bit more every month.
I can explain more over skype whenever you're online.