A: Storage capacity affects only amount of files/projects you can store in the system. You can delete the old projects to free the storage at any time, but if you want to have a project history for years, you would want to have more storage.
My personal choice, even if you don't have lots of employees, is tier 3, just because it has 15 users and 30GB of storage space at a price relevant to our monthly fee for the Pro XL plan.