You could try that Songsmith program (http://research.microsoft.com/en-us/um/redmond/projects/songsmith/) and see if you get background music that roughly fits what you're trying to do. It's not expensive at all ($30). It uses MIDI and not the greatest quality sounds, so it would probably sound cheesy at first. But you'd get an idea as to whether you're on the right track or not. You could then export it to BIAB, tweak it, add RealTracks, MIDI soloists, etc. Export those tracks to a DAW (RealBand, PowerTracks, Sonar) and redo the vocals for a final product. If you use PowerTracks, you can even generate background vocals - a feature that BIAB has too.

BIAB has a lot of bass, acoustic and electric guitar, and drum sounds, so you will probably be happy with what it has.