SAN

What type of SAN features do you want? What type of support do you want? Does the company need to have a long track record, or is a startup okay? Are you looking at at tier 1, 2, or 3 vendors? What about homegrown storage using off the shelf hardware and software? What type of budget? This is a very interesting time in storage, and cloud storage is becomming viable for many needs.  Answer these questions and we can give you some resources.

You can create a 5TB share and put it on a SAN for under $1000.  It can handle 1,000+ PCs, as long as they aren't doing much.  Or you could spend $100K.  
How about some constraints?   I take it you already have at least 100 users sharing something ... that would be a good baseline.   Describe what you have and say that you feel it handles X number of users.

Otherwise this exercise will be a waste of time for all, because have describe the proverbial, I have 200 users who are hungry and I need to buy food.  Can you please advise what is the best food with pricing? :)

The other thing to consider is... How are you going to protect the data from a failure?   5TB is going to take a lot of DVDs or many days to backup up to an online service (and many days to restore).

How long can you afford to be down if the storage fails (seconds, minutes, hours, days)?
How much data can you afford to lose if the storage fails (a transaction, a second's worth, a minute's worth, an hour's worth, a day's worth?)
How long do you need to retain backup data?   (weeks, months, years, forever)

5TB of data is past the point of slapped-together backup solutions, IMHO

I thought SAN is raid controller with Fault Taulence of a disk.
Users share data files. And it will be used for backup storage as well.
We cannot afford downtime.