I have though about this alot.. You need to determine what internet services you want to provide. Is this going to be something that you want to lean toward college kids doing research papers or gamers or business people on thier lunch break..
With the exception of gamers, you really don't need very powerful PCs. They aren't going to be doing anything that requires that level of performance.. Word processing and internet services (www, ftp, email, etc) don't tax a system that much.
As to security.. I was thinking of using swappable drive bays and having the option of a customer renting a hard drive by the month, and including a liberal amount of time with that rental.. However, it would be adviseable to use distributed applications, something like Lotus notes so that the customer can't access anything other than the program menu (which would have only links to server side applications) without "checking in". Charging the rental by a recurring charge to a credit card and it is a fairly reliable monthly income.. This is an option if you have a large college very close by (as I do). This would allow the college students who don't have computers to be able to do what they need to do without having to deal with going to a computer lab that might not be open at 1 in the morning the night before their term paper is due.
In addition, even though you are setting up swappable drives, have them log into a server and when they check it, you set the access control for the server (I know this is possible with 2k server) for the amount of time that they pay for. If they need more time, they have to come back and pay for more time.. The good thing about 2k is you can set it to give a message (not sure on whether you can adjust the timing or not) that will let the user know that in a certain amount of time the computer will shut down and they better save thier work.. If they come up and pay for more time, you just change the settings..
As to jumping on the ticket... They have to check in and pay in advance, the computer shuts down when thier time is up so that isn't a big consideration.. Also, for the "casual user" who comes by for a snack and wants to read the headlines.. A section is set aside for those customers.. The waiter/waitress comes to the table and they place an order for the food and the computer time.. You then "turn on" the computer by allowing it access to the network for the amount of time that they ordered from the waiter/waitress.. If they try to jump on the bill then the cops can be called and it's just like jumping out on a restaurant ticket.. which means big fines and possible jail time..
Access control for pron.. Just lock it down.. There is NO reason for anyone to surf porn in a public location.. There are many quality server side "nanny" type programs that will let you allow specific sites that would normally be blocked (good for those pesky medical students). That and making people who request that consideration show ID for proof of age should be sufficient.. Not to mention that if you do that, mom and dad will have no problem letting jr come down on a slow tuesday for a lan party (should you chose to do that kind of thing).
Personally I wouldn't get too in depth into the gaming scene. Most of the kids who would want to play don't have any money and those would be the one who would try to skip on the bill. A sponsored and well advertised lan party from time to time with an entry fee would be fine, but adults don't want a bunch of teenagers and preteens running around when they are trying to get work done or shoot off a business email.
As to after hours security.. That's easy.. ESPECIALLY if you have a college nearby.. Don't close.. a 24hr computer lab for college students with food readily available would be a godsend to ALOT of students.. I know it would have helped me immesurably as I was too poor to purchase my own computer, but would gladdly have spent $20-$30 if I could have walked in at 3 in the morning and gotten 2-3 hours of work done.. If you can't stay open 24 hours.. INVEST IN INSURANCE AND A GOOD PHYSICALY SECURITY SYSTEM!!! With that many computers laying around it is a very tempting target for the unsavory element of society. Paying insurance premiums suck until you are left out in the cold because some jackass descided he needed another crack rock or a new pair of shoes..
And the biggest thing. REGISTER YOUR SOFTWARE and make sure that, in the case of server side applications, that you have licences for each and every seat that has access to those applications. The amnesty is over and the D.A.s office will hammer you..
Hope this helps,
Alexander