It's hard to answer any of those questions w/o knowing more about the application. Bandwidth is a function of page size, # of users, # of page hits per user, time between page hits, etc.

The number of web servers and load balancing is also a function of the above questions plus how much time each page hit takes on the server.

The amount of storage depends on what you're storing, how much data is being added to it, etc.

Is this app. complete? If so, your best bet is to have someone use it the same way as a normal user and log everything (page size, page render time, # of hits, time, etc). You can use this to help extrapolate for the load.

For a backup solution, that's mostly a function of budget, the cost of being down, whether it can be a warm standby or hot standby (for immediate fail-over).