Windows, ideally a pro version so you can lock the systems down. And Firefox for web browsing.
Or, you could save much money on licenses etc. and try Ubuntu with Firefox for web browsing. But the Windows option will be easier for your customers to use.
Since this is for a business and security is very important, I would highly recommend that you hire a networking expert. There are so many things to learn that come from experience and setting up lots of networks.