With a Caldera or Onyx Thrive you will not only get a RIP-software, but a collection of basic printer profiles too. Thats important to start with, except you are able and willing to do profiling yourself.
You could get an Onyx Thrive 30-day full functional demo with which you could start (download at
www.onyxgfx.com). Caldera runs on Mac and Linux only. I think it is important to start running a FB550 before investing in software: if the machine isnt in good condition, this could become a real money pit.