Depending on which version you purchase, the number of styles to choose from can be considerable. I think there are around 1400 styles at this point (maybe more, don't know without looking but it is a bunch). Most are based on styles of particular artist and/or genre. I would guess Johnny Cash is in there. The style determines the overall feel of the song along with the tempo. Once you have the chords for the song you want to input, typing them in is fairly simple with only a few rules, such as only 4 chords per bar can be entered as BIAB stands now. Putting the song into BIAB is a matter of typing chords and selecting the style. Since you have been reading the forum post for a while now I guess you know about RealTracks and RealDrums. They have (in my opinion) greatly increased the final product you get when you are done tweaking your song. Like Sundance posted above, it isn't hard to get started and once you hear a few bars of your song you will be hooked and want to explore the program further. It is just so much fun and perfect for what you want to do. Give it a go, and the 30 day money back policy is your safety net. If you don't like it, send it back. I bet you won't, though.
Good luck and welcome aboard.