You might consider going full commercial and financing a portion of the cost. Right now, you can buy many commercial mowers on a 48 month, no interest deal. I'm not advocating taking 4 years to pay for one, but rather than limit yourself to a maximum of $6,500, you could take advantage of low or zero interest and purchase a mower that will last you for years. The time you will likely save by going with a 52 inch full-commercial unit would likely allow you to add at least a couple of yards a week, making it relatively easy to pay off the difference.
By the way, if you have a dealer in your area, you might want to check out Gravely's.
Dependable Mowing Service