You can directly implement Modbus RTU with Xbee modules in 2.4GHz, 900MHz, 868Mhz. Higher frequencies are more directive in nature and requires line of site operation to minimize packet loss.
I guess you will implement this in an industrial environment. If so, you should be concerned about the mesh network design : to configure appropriate number of routers, coordinator devices and the end devices. Consider their positioning on site with possible fixed and movable obstacles, canopies etc. Consider every building as Faraday Cage : http://www.thedailysheeple.com/wp-content/uploads/2014/08/faraday-cage.jpg
Is powering the nodes is an issue? If it is then you should calculate which units will have more radio traffic and ensure more power for them.
If you consider XBee-PRO 2.4GHz embedded RF modules from Digi International, then meshing will be automatically establish by their propitiatory DigiMesh protocol. Read more : http://www.digi.com/pdf/wp_zigbeevsdigimesh.pdf
Zigbee is one of the wonderful protocols to work with. As you are designing the product yourself, instead of using modules you can also implement it as a stack.