Whereabouts in Michigan? I'm in the Clarkston/Davison area. While I don't agree that Michigan has much for hills (born in Va, have lived in Id ) I'd say to start with riding the mountain bike. Get some 35-45mm size tires with a low or file tread. That will easily cover gravel, dirt and pavement. We have lots of great dirt backroads that are lightly traveled and those tires will eat them up with the right pressure to smooth out the washboard and smaller potholes (dodge the big ones). From there, figure out what works for you. I personally like a drop bar with some flare to it (Randonneur or Woodchipper bars) and a fat tire. There's really so little gravel on the roads around me that it's almost like road riding on a dry day.

Head over to the Recreational Cyclocross and Gravelbiking forum and take a look at some of the pic threads there. Lots of different solutions to the type of riding you are looking at.