You should hold off on creating your network plan until you have truthfully assessed your networking needs. Once you have that information available, you can start the task of determining what kind of client machines you need to purchase and what kind of servers you will have to set up on your network.
Keep in mind that business networks must meet the business requirements of your company or institution. While it is very difficult to assess the actual monetary value added aspects of business computer networks, the network must fit into the overall business plan. This relates to budgetary issues, the infrastructure of the company (do you have to connect remote offices to the central office?), and the level of security required for the network. A company handling credit card information on its network will certainly need a higher level of network security than other networks.
A good way to expand your understanding of network planning is to read through this book. It provides nuts-and-bolts information on network infrastructures and some of the choices for network clients and network operating systems. Because this book does not profess to be anything more than an introduction to the world of networking, you will need to do some additional research. The World Wide Web is an incredible resource for the novice network builder. It provides information on products, networking theory, and even allows you to purchase the hardware and software that you need to get your network up and running.
In situations where you will be involved in the building of large networks, you might want to go outside your company and hire a consultant to help you plan the network infrastructure. In the case of smaller networks, a little research and some careful consideration should help you put together a network that will meet your needs.