Looking for opinions on small business storage options

I own a small contracting business that has me and a part time office employee that accesses various data during the day, located in a separate office from my main residence, and would also be accessed from the computers in the home after hours.

Other devices are 3 desktops and 2 laptops all running W7 Ultimate.

Here is what I am thinking of doing.

I have my desktop that I am rebuilding with a i5-3570k and ASUS P8Z77-V PRO board, 16 gigs memory, gtx470, Antec P180, 750w Psu, and Win 7 Ultimate. This system runs 24/7 as I access it remotely.

I currently have 3 - Seagate 1.5 drives as well as an older 320g drive.

My thoughts are to install a new 500g drive for my op system, and at least 2 of the drives in a raid 1 config and purchasing a back up software to back up the raid to two locations. One is on the network, in my house, and the other to be online somewhere.

I do have an issue on the reliability of the drives. I have all three listed as Caution with CyrstalDisk due to reallocated sectors count. So I am open to replacing them.

We have a 3 QB company files that would be on it as well as all documents, pictures, music and video from all other computers.

Is this a reasonable plan?

Would a NAS be better and why?

I am not sure if if this would be enough storage as we are moving to scanning all invoices and etc as they come in. I also keep on hand all pdfs of the projects we bid ( 6-12 months if we don't win the bid).

Would I be better of to purchase a 4th 1.5 drive?

I am looking for redundancy and a logical back up plan.

Any help/guidance would be appreciated.