Casey, I think a good commercial 32" to 36" walk behind is one of the best mowers a start up business can have. It is small enough to fit into most areas, but still large enough that you can handle larger jobs. I also think a small walk behind leaves most areas with a nicer cut.
My former business began with a focus on custom landscape installations, and we started out with a 36" Exmark walk behind to handle the maintenance customers we wanted to keep. As the maintenance side of our business grew to 2 and 3 crews, we still chose walk behinds because of the quality of cut. Yard size dictated that we purchase 52" and 60" mowers, but my personal favorite mower was still the 10 year old 36" Exmark.
I am sure you will hear other valid opinions, but if I was in your shoes I would start with a 32" to 36" walk behind.