When buying a franchise, should you stick with the industry you know?
The franchise company will have set up training programs to teach new franchisees whatever they need to know about the specific industry as they prepare to open for business. They will also have designed marketing programs to attract customers based on the franchise company's knowledge and experience in the industry. This is one of the great advantages of franchises as business opportunities.
You also asked if you should focus on a "simple" business and that's a fantastic question. The answer is a resounding YES (though you also mentioned two food related businesses and most experts will tell you that there is no such thing as a simple business involving food).
It is hard enough to succeed in any new business and you want one you can learn easily and that doesn't have too many moving parts. The more complicated the business, the easier it is for you to make expensive mistakes while you're learning it so remember the KISS (keep it simple stupid) principle when evaluating opportunities.