There are some guidelines (like 1Mb memory per user on the server), but these are usually incorrect, as it all depends on the application and usage. For example, network depends on number of concurrent users and size of the pages. Storage depends on the data you are storing, files that are uploaded, any other documents that you create, etc...

Best advice is not to buy hardware based on expected application requirements. Make sure your hardware scales, and use virtualization to adapt. Requirements of your application will probably also change during its lifetime. Generic server hardware, 8-16 cores, plenty of memory, will allow you to host a number of virtual machines. Scale virtual machines as needed. Add memory and machines if you run out of hardware to scale your virtual machines. Virtualization allows you to adapt fast if needed, ordering and installing hardware usually takes days to weeks.