Build a secure network for a small business ran out of my home.

Hello, I have been having problems with network security at home. We have just started a small business and want to make sure our home network security is up to par. I am hoping to create a network that isolates a work computer that will only be used for accessing our website, banking and business related tasks. The other part of the network will be used for day to day email and web browsing. The business computer will be a pc. The other computers are macs. I recently purchased a zyxel Unified Security Gateway. It is called the ZyXEL USG 100. It has two wan ports, 3 lan ports and one port for DMZ. We are currently using ATT dsl. I was planning on adding another line for the business computer with a static IP. The other wan port would be used for the day to day non business uses. The business computer will only be able to connect to the net through ethernet. It wont have bluetooth or wifi. This could be a terrible approach. I was hoping someone with some professional experience could get me dialed in and come up with a secure network. Thanks for your time