First of all, make sure you get a nice fast broadband internet service, not just some average 512k like you'd get at home.
Secondly, IANAL, but have a sign or something basically as a disclaimer saying that anything you use the wifi for is their own responsability. legal action may be taken, etc.
Also, the router you'll use to let people connect to will be able to block ports. Let them know on said sign that the service is for web and email only. So you'd only want to allow data through say ports 80,20,22,443.