I'd say no to the riding mower but not sure $3500 is enough to get a good zero turn, even a used one. I see you have a 36 inch walk behind with sulky. Honestly that's probably as efficient or more efficient as most riding mowers.
What's the situation? Are you having a hard time getting everything done with the 36? Too many accounts? Accounts too big. A little more information about why you're considering adding a mower would be helpful.
My first instinct is to suggest you get a used 52 inch hydro commercial walk behind. That's usually the next step up and along with a 36 is a nice combination. The Exmark Turf Tracer HPs are a nice or you could stick with Bobcat and try to find one of them. They will take the same blades as your 36, just 3 instead of 2.