First, I'd never use a wireless connection for a server. I'll always hard wire the server to at least a switch and use that for other machines that are stationary as well. I still believe that wireless connections are not stable enough, nor secure enough to be used for a valuable resource such as a server.
If your office is not already cabled, unusual in this day and age, get some quotes from people who do cabling for networks, and get your office cabled. Then you should set yourself up like this:
1. Incoming Internet connection.
2. Router. Many routers now include a firewall of sorts. some are very good, some are pitiful. You will need to make a decision here, use a router with a firewall built in or use a standalone firewall.
3 If you use a standalone firewall, this would come next, else skip to 4.
4. Switch. Make sure it has enough ports and some extra for future expansion.
5. a. All of your cabled equipment should be plugged into your switch. This means computers, printers, and other devices that need a connection to the network and/or Internet.
b. Your main wireless access point. There are a number of ways you can go about this. Depending on the size, configuration and construction of your office, You may need to add other access points. These can also be hardwired, or you can develop a mesh network, where only one access point needs a wired connection point, though, for redundancy, you would probably want at least two that have a connection to the wired network.
You will want to use your current server to offer DHCP as well as DNS services for your network. If you have not already, you will probably want to create your own internal domain, using the current server as a DC (domain controller) so that you can join employee machines to the domain, and guests can still access the Internet directly without joining the domain. Depending on how you set things up, they may or may not be able to see your server anyway. With most mesh networks, you can have both a public network for anyone, and a private network for your employees to use.
If you are having a problem understanding what I am talking about, then you should seriously consider getting a network guy in there to advise you and help you set everything up so it gets done properly. It would definitely be worth the money.