Definitely get a first aid kit that can cover most environmental hazards in your area (spider bites, poison oak, etc) as well as the basic cuts/scrapes/twisted ankles/etc. Don't forget to include sunscreen and hydration tablets, since most of your player base is likely not used to spending a lot of time outdoors. Make sure you know how to recognize and treat heat sickness, dehydration and over-exertion. Getting first aid training yourself, if you don't already have it, is also not a bad idea.
It's not necessary to have your event catered. Find out what your cooking facilities will be like and if open flames or camp stoves are allowed (some places in high fire risk areas will only allow small propane camping stoves; other campsites permit open fires or charcoal grills). Let your players know that they should bring and prepare their own food on site. If you'll have access to a kitchen, see if there are any players who are willing to cook for the whole group. Then just have everyone pitch in $10-15 as part of a group meal plan.
As far as drinks - water water water water water. One gallon per person per day. Drinks like VitaminWater, fruit juice or Gatorade are also a good idea, but water is the most important.
If you're planning a weekend event, you'll probably want running water and showers. For a day event, you can get by with just toilets. One toilet, plus one more for every 10 or so attendees is a fairly good ratio (at least two, because if one breaks, you want the other one as backup).
Look into campsites in your area - national and state parks, any sites your county or city might maintain. One of my larps regularly plays at a Boy Scout camp, and the Girl Scouts also have their own campsites. If we play on a weekend where the Scouts aren't going to be there, we're able to rent the site for a not-too-shabby price.
Lastly, you will need insurance. If you're running the event, and something happens to either a player or the site itself, you could be liable for any damages or medical bills. So get insurance. You can generally find a pretty good policy for around ten dollars a head, less if you sign waivers to disallow minors or alcohol. And that cost gets rolled into the ticket price for your players.
There is more information about logistics on a site I run.