Capacity Planning

I have a total of 10 departments with 100+ users in company. I need an advised regarding proper implementation of file server for important data only. What is the most easiest and quality way to develop a sort of capacity planning from now till the next 5 years or more. Is it proper to let users save on a dedicated server? or we deploy a sort of data storage? I also want to implement this with security and back-up system. Thank you for any advised:)