I would highly recommend that you spend some time driving around and seeing what the competition is using. While you'll see a variety of brands, you will also see just a few over and over again. Here, where we get about 4 ft of rain a year, have a ten month growing season and lawn care is a HUGE industry, there are two brands that I see all over the place. Exmark and Scag. I went with Scag and love it. I looked at Exmark and was really leaning towards the Kubota, but nothing really compared to the build of the Scag. I really liked the water cooled engine too.
Since you are doing this profesionly, be sure to stick with commercial grade mowers. Some of the ones people mentioned are more for estate mowing and not meant for five days a week, constant use. They are also more comfortable then a commercial mower, but with comfort, you give up a degree of strength.
Even if you are looking at used mowers, go to the dealer and ask them about their mowers. Dealer support is always your number one concern when buying any piece of equipment. It's even more important when you are using that equipment for your income. If a part breaks, you are not making any money until it's fixed. Plus you are more then likely annoying your clients. It's a simple thing, but one that puts allot of guys out of business.
One of the things I really liked about Kubota is that they are about as good as it gets for parts and service. My Scag dealer is also my Echo and Stihl dealer. I've never needed any parts from him for anything, but have always been able to get get chains and supplies from him. A good inventory is nice to have. I go there, and he has what I want in stock. It will be interesting to see what happens when I need a major repair.
Good luck, like any business, it's all about doing what you say you will.
Eddie