Right, you don't have a problem with speed now but if you're going to be marketing and picking up customers it will become an issue. Part of the problem with a lawn tractor is the amount of trimming it leaves due to it's poor maneuverability...yeah I know it only has a 12 inch turning radius, but it's a pain to get close to stuff. It's not just top speed I'm talking about, it's how quickly you can mow a lawn...overall productivity.
Look around at the pros cutting lawns in your area....what are they using? How many lawn tractors do you see? I'm suggesting that you started wrong and that it will affect your ability to be optimally productive and your ongoing payments for the wrong equipment will keep you from getting what you really need, in the near term at least. Now if you already had a lawn tractor and a small utility trailer and wanted to start off with it, I'd say sure, go for it until you can pick up something more suitable, but I sure wouldn't recommend taking a loan out for a brand new one.
I don't mean to discourage you but you aren't off to a good start from where I sit. You have 8 customers, a slow top heavy homeowner-type mower that isn't rated for the duties you will need to put it to in order to be successful, you're in debt, and you're business can't operate year round. Is it hopeless? No, I don't think so, but I think you need to get away from the thinking that your next purchase should be a zero turn rider. I think you need to get a walk behind, loose the tractor, and then when you have a full schedule and can justify it, add a Z.