What's different from buying a house?
You are buying a BUSINESS, not a piece of real estate. Your due diligence has to go well back into the history of the business, past and current financial situation, past, present and future business prospects.
It also goes a whole lot deeper than most buyers realize, and buyers often fail to appreciate the value of management skills and goodwill, and what is working for the current owner may not work for the buyer. This gets really hard to quantify. But a thriving business may not be a thriving business under a new owner.
My father sold his thriving business (collision shop) and the buyer went broke within five years because he lacked the personal skills to keep his regular customers and the experienced employees. My father-in-law sold his share of a partnership to his non-operating partner, who took less than two years to go bankrupt trying to run it, again because he lacked the personal skills and lacked the business sense.