That price point is too low for what you ar looking for. You will need at least a good quatly 4 disk NAS and with that storage capsity you are looking at about $900-$1000 at a minium.
a good NAS for 5 users $530http://www.newegg.com/Product/Product.aspx?Item=N82E16822108122
4 3 TB disks about 9 GB of storage (using SHR) $560http://www.newegg.com/Product/Product.aspx?Item=N82E16822136874
1100 total and i think a good confguration for your use. if you go this route ensure you are using a Gigabit connection between your NAS and all computers in the network switch included.