If you do NOT plan on reselling the wi-fi service, then the rules are MUCH different. If you resell it, then the ISP has a different contract. If you look at the standard cable modem or DSL contract, it specifies that you can not resell it. Don't even tell them what you are doing or they will want to convert you to a different contract even though a 'free service' is not a violation of their standard contract. Under those conditions, a local 'knowledgeable' PC guy will be what you need.
If you are planing on reselling, then you better contact the ISP and expect to loose money. Now, you have to have them or yourself provide more hardware to track usage. You have to pay the ISP more money becase they want their cut. And just the headaches of managing the system will drive you batty.
To be honest, setting up a wi-fi is SIMPLE. If you can mount an antenna on a pole and run the cable, you can do it yourself. You don't need a PC guy. And if you want to prevent non-campers from using the wi-fi, just disable broadcast of the SID and tell any camper what the SID is. And change it about once a month.
You want to play with a simple setup to see how it works, just go buy a Linksys WAP54G and plug it into your cable modem or DSL modem. Set the box just inside the window to the pool deck. You now have free wi-fi for those that want to use it at the deck (and in the laundry room if very close) and even some of the closer rigs can use it. All without a large external antenna and under $100. Follow the instructions or ask a few questions here. It really is that simple.