I suspect more than one person will recommend you buy a walk-behind, rather than a Z at this time. I've never run one, because I was in the position to purchase a new Z for cash when I started my business, but if your budget is a problem, you can get a lot more bang for your buck with a walk-behind. You could buy one new unit, with a warranty, or even consider picking up a couple of used units, so you would always have a backup.
If the WB route doesn't appeal to you, I'd consider spending more money, if you are financing anyway, and going with a full commercial unit. If there is a Gravely dealer near you, the 100 series of their Pro-Turn model might fill your needs nicely, and would not be all that much over budget. Also, check and see if your Gravely dealer has signed up for their new program whereby if your mower breaks down, they will give you a brand-new demo unit to use until yours is repaired.
If no Gravely dealer nearby, there are many other makes that would meet your needs as well. Good luck.
Dependable Mowing Service