I guess I would highly recommend going and running a band mill for a couple of days before you take the plunge. Do you like working in dusty conditions? Do you like the constant drone of loud air-cooled 4-stroke engines? Personally, I don't like the work environment a sawmill creates. And don't get me wrong, I like the larger chips and intermittent noise from chainsaws.
Another thing to consider is the market, as others have mentioned. Figure out what your costs are to run the mill on an hourly basis, blades, grease, gasoline, air filters, etc, and don't forget depreciation and insurance (I'll come back to that topic), then add your desired wage, which should be at least $20/hr. Now, think long and hard about whether anyone will pay you what you need to make money. When I was trying to make some income with my B20 I was turned down many many times when I was charging $50/hr, and that is a fully hydraulic portable mill with more HP than an LT15. I honestly think that if you want to make money with a mill, you have to sell lumber or something more valuable than that. Custom work is too much hassle in most cases; customers are flaky, want to talk you down, want you to fire up for one log, want you to saw their yard trees, didn't read where they signed that they'd pay for ruined blades because of all the metal in their yard trees, etc. And if you're talking about milling on-site there's even more room for losing money; there's time wasted driving there to give a quote, etc.
And, back to the insurance topic: this is a BIG PROBLEM if you want to run a mill. I don't know who your insurance is through, but tomorrow morning, call up your agent and tell them about how you're going to start a sawmill business. Chances are that they'll be ready to cancel your entire policy. That's what happened when my agent saw my mill. Even when I told them that it was there for storage and that I only used it on customers' properties. It didn't matter. You could sell 300 cords of firewood a year and have all the associated equipment, but you say the word "sawmill" and everything changes. I had a darned near impossible time finding someone who'd even consider insuring me while operating a mill, and it wasn't cheap. And it's not a business that you'd want to run without insurance either...
So, that was my experience in buying a mill for some side income. I hope you make a more informed decision than I did.