I have the impression that it's best if you look at something just created itself, with popular Windows applications such as Excel or Open Office. Then you have at the beginning, although a lot of work, but if the files internally generated have proven once, they facilitate an all settlement also, and you can always improve a bit over time.
My advice: buy this software once, learn from it, what a landlord so must know everything, and to create its own files for billing based on this experience.