Round bales or square? Is the land to be hayed flat or hilly? A square baler can really jerk a tractor around. The issue with baling isn't just the hp of the tractor, it's also the weight of the tractor. Having said that, there are some threads in the Kubota forum, with pictures, of haying with a tractor about the same size as a TC45DA. I'd also ask your dealer what he/she recommends. They should know what size tractor and equipment would work in your locale. Have you ever baled hay before? I ask because it's a hot, itchy, tiring job that can not be put off. If doing square bales, do you have people who can help you? Got a place to store them?
How much $ do you want to spend? What equipment do you already have? Hay rake, wagons, mocos, bale spear, etc.? Could end up being a lot of money, not to mention a lot of work, for only 15 acres, unless you buy used. If you buy used, how handy are you? How close is a dealer for parts when you need them? Even then it could be a decent chunk of change. Might be easier to have someone else do it. Do you work full time and will be haying in your spare time? If so, can you take off with short notice when the hay is ready to be cut, raked and baled? Would you like working a full time job, coming home, wolfing down a meal (maybe) then spending four, five or six hours on the tractor?