If you will be stalling horses, bedding costs an arm and a leg, too. With 15 horses, our bedding bill is over $400 per month (we use baled pine shavings - like the consistency and scent). To do this all yourself while working full time will be a major strain on your personal time and family life. I can't stress this enough. What does your family want? You havn't mentioned that, at least that I recall. Also, what exactly is your horse handling experience? You need to consider whether or not you'll allow stallions. If you do, will you charge more? Whether or not you'll allow young'uns who may not be trained? What level of training do you expect the horses you handle to have?
I'm not too familiar with Kansas, but don't you guys have pretty cold winters to the extent that you'd have to worry about water freezing over? In Mi, where it is below freezing for 3 months straight (that is a low estimate), we can count on our electricity bill being over $900 a month with tank and bucket heaters running ONLY during the times the animals are in the stall (bucket heater) and outside (tank). You can cut costs by dumping/refilling tanks throughout the day, but you'd basically need to be at the farm ALL day to make sure you don't leave your horses out without water. In the summer we are consistently about $190 a month, so it more than quadruples our bill!