Need suggestion in choosing an effective wireless Transmitter and Receiver
XBEE (XBEE Pro to be precise) is a good option if you don't have obstacles in between... XBEE simulates Wired communication over Air. All you need to do is get Two XBEE Modules. It's same like communicating over RS-232. Whatever you'll write in Your Serial Buffer, it will be transmitted over wireless medium. Moreover, there are some Readymade protocol stacks available in market by which you can do Mesh Networking. 

But Keep in mind that, XBEE will work for the distance you mentioned only if it is obstacle free or less obstacles.