Bernie: unfortunately RPi900 won't work with the B+, due to the removal/relocation of the P5 header and mounting holes. You'd need a model B or model A. The model B (at least) should still available.
You only need the radio module itself (DNT900P) to use with RPi900. I usually get these from DigiKey. (Mouser and a few other places also have them.) RPi900 boards are available from http://rpi900.com
If you buy the manufacturer's DNT900 developer kit (DNT900DK), this includes two radio modules and two interface boards. You wouldn't need RPi900 boards in this case; you can plug each interface board into any Raspberry Pi (including B+) using the USB cable. The developer kit board appears as a standard USB-to-serial device and you can talk to the radio over /dev/ttyUSB0. The DNT900 line discipline I wrote works just as well with the developer kit boards over USB.
The main benefits of RPi900 are the compact form factor, and the on-board power supply, real-time clock, status lights, I/O connectors and remote shutdown circuit. Much smaller than a Raspberry Pi plus developer kit board; great for stuffing into a small outdoor enclosure.
As for software setup, I support Arch Linux and provide packages to set up PPP networking fairly quickly. This includes a star network topology (or point-to-multipoint, in DNT900 terminology). On the Raspberry Pi base station, each remote radio appears as a separate tty (/dev/ttyDNT0, /dev/ttyDNT1, etc). The base station establishes a PPP link to each remote over these ttys. It provides IP forwarding and proxy address resolution to the LAN (probably via ethernet). The end result is that the remote Raspberry Pis are connected to your LAN: you can SSH to them, serve data from them, whatever you want. See the RPi900 website for more detail on how to set this up (a few lines of configuration are all that's needed).
Feel free to email me if you need any more technical advice!
RPi900 - long-range wireless for Raspberry Pi.