No problem - just use wireless security. (WEP WPA or WPA2)
You should contact a company like Verizon, who can set your small business up with a network with their IT departments. A Wireless USB network is often places in schools, so getting some info about certain school networks could help you in your search.
"You can purchase a wireless network card at any Staples store. And as Staples has many locations and a website, buying the wireless network card should be easy through them."
This is illegal and you shouldn't try. If you own the wireless network, it should be provided by the network service providers!
If you are concerned about information security, wireless security protocols add some small level of protection. If you are not concerned about it then encryption is an extra level of hassle.
Yes! You can install a wireless doorbell with an ordinary wireless network router. This is a very common practice and the company who sold you the wireless doorbell should be able to help you set it up.
Every computer that is connected to a local area network or LAN should have an Ethernet card. For wireless networks, computers should have a wireless adapter.
A wireless card for laptops is used to enable the laptop to connect to a wireless network. A wireless router is used to keep track of the whereabouts of devices connected to the network. It is part of the network. This is a very basic answer but I think it should be enough to answer your question.
802.11n and 802.11a
It needs a wireless card or it should be connected to computer connected to the wireless router you are using for your network.
DNS
Unsecured wireless network without MAC filter.
Disable the broadcast of the SSID on the access point.