The UC560 supported up to 138 users, but has not been available for purchase since the beginning of the year. You can certainly get them second-hand or used, but hardware support for the device will no longer be available after next month.
http://www.cisco.com/c/en/us/products/collateral/unified-communications/unity-express/end_of_life_notice_c51-729017.html
Cisco 2960 switches work fine in a UC560 environment, but you ideally want PoE models if you're connecting IP phones to them.
Licensing is done by user, but with the product having reached end of sale, you'll want to make sure that the licensing is still available for purchase.
Additional components may be needed depending on what kind of deployment you're planning. For example, if you're using PRI to connect to the telephone company, you'll need to make sure you have a PRI interface on the unit.
