Let me elaborate on this.
Firstly, you have to make sure that if you invest in a commercial grade ZTR (about 4-5K) it will be used enough to be payed in the years that it will last you (amortization).
Don't forget that if you go into commercial stuff, they are bigger and need more maintance. Leave clean ups will be longer, trimming might take more time and it goes on. Make sure that if you go into commercial mowing, you will want to also have enough time to do all of your residential accounts.
Secondly, you need insurance for this kind of stuff! A cracked windshield on a CEO's BMW will not be good. The same goes for that little pebble that flew into the employee's face (luckilly we are in Canada, so they cannot sue you for enormous amounts!)
So, what you should do is sit down an assess the situation. Calculate how much it will cost for a ZTR, insurance and whatever else you need to do those big commercial lots.
Then calculate how much it will you cost to invest a little more to be more productive in the residential area. My recommendation is get a person to work with you! Once you have this person, contracts can flow in with no problems and you can take on the challenges of the demands.
Being in business is always about riks! You have to take some! There is never an answer, but a new question.