To begin with, congratulations on starting your business and I hope you keep up your enthusiasm... it will help get you through all the rough spots.
You may be noticing a pattern here... the guys with all the experience don't seem to focused on what equipment to buy, and you've got your mind set on that zero-turn! Well, there's a good reason...
The equipment really doesn't matter. Get the best professional gear you can afford that can be serviced by whatever local shop you like working with. Sit down with an accountant so you can depreciate any expenses... you will probably be rolling your profits back into the business for the next couple of years and won't have to pay much in taxes.
Your real focus should be on customers and customer service. Work on expanding your customer list in the areas you are already working, and offering a wider range of services to the customers you already have (getting more juice out of the same orange!) I have found that the lawn cutting returns a modest profit, but most importantly gives you access to the customers for the extras -- and that's where the good money is.
Do everything by the book when it comes to insurance, licensing, etc. And with every decision you make, look at it with a cold eye and only one criteria... does it put more money in your pocket at the end of the day? Shiny new machines get old fast; trucks are great until you need your first and second transmission, brake job, etc. The best asset you have is your customer list and the relationships you have with those clients.
I started 20 years ago with a 21" trimming mower cutting 11 lawns a day... and did myself a big favor when I could afford my first 48" walk-behind. I realized there was a limit to how much money I could make with my own two hands -- and for a lot of guys keeping it simple is worth it -- but the only way I could keep growing was to get away from doing the labor and into running a business.
For reference, I now cut 200 lawns per week with 15 men (I've had as many as 36 employees, but I wasn't making any more money and the headaches were unbelievable!) The most profitable division is in new landscaping. Don't be afraid to subcontract the work you are not skilled or experienced enough to provide... you can start up relationships with local masons, irrigation companies, etc. and make a little profit on their work. As I said, it's all about relationships...
Again, good luck to you