An opportunity?

We have recently relocated to Tennessee and there is a local business for sale that has peaked my interest. The business has a house and 13 large greenhouses on a few acres of land. The seller is 80 years old and wants to retire. In the past, he had been selling fruit and veggie (annuals) starts in the spring as well as decorative flowers. He sells year round decorative hanging plants, cactus and some other seasonal items. This is not any kind of permaculture set up. It seems to me that his asking price is reasonable (and negotiable). My question here is how do I look at developing a business plan to not only purchase but to convert this over to my vision of using the whole operation as a small farm. I can't wrap my head around how to do any kind of due diligence since how the business is now isn't how I would be running it. My husband is not the visionary that I am and is hesitant to go any further until we have some idea of numbers. I need help figuring where to start to gather the information I need. Any personal experience, book/video recommendations, etc appreciated. Thanks.

I think Daniel Kern in this thread gives a good example of what a good business plan document looks like. Also, the Tennesse government has a nice guide online for small business start-ups. I know some people don't like it when they are referred to the Small Business Administration, but I think it is worth checking out the wealth of information they have available.