I'm no expert, but I suspect you will need some kind of insurance regardless. Property insurance as above and some kind of liability insurance. It's all well and good players signing waivers, but what if someone falls through a rotten floor or something? I'm sure you'd be found negligent for not checking the safety of the site. You'd also need it in case some random member of the public wanders onsite and gets shot in the face - they won't have signed a waiver.
If you were charging people, it would probably be seen as a business. There's probably all sorts of legislation regarding change of use of the site from industrial to leisure. If there are houses nearby you'll have to account for noise polution, extra traffic, parking, people wandering around dressed like Rambo etc.. With the current security situation I'd imagine you'd need to inform the Police of your intentions as well so a well meaning member of the public doesn't place a worried call about an ISIS training camp in the area. I reckon Police MP5s hurt more than my CYMA ;-)
I'm sure it can't be as easy as you think, or CQB sites would be springing up all over the place. It will probably take a lot of research and cost a bit!