Dealer support is what should matter the most imo. Most big brand commercial machines will get the job done. All have different features and designs. I would look for a Kohler or Kawasaki engine and as long as you don't have to worry about small gates and hills then a zero turn will be very productive. A 36in walk behind is what I use for the small back yards or lawns with a hill and I use a zero turn every where else I can.
Good luck in your new business.