Would the first two weeks in October be too late in the season for the Black Hills area? I'm thinking mostly about the weather and driving conditions. We would probably also go to Devil's Tower and then into North Dakota to Bismarck too.
I used to live in Minneapolis and I suspect that the weather is roughly the same. Early October can be delightful. Or it can be cool/cold. In other words, it can be variable. I googled Keystone, SD weather (where the Black Hills are located) and it said: October Min: -3°F warmest: 81°F Average: 41°F Another website said: October averages a high of 60˚ and a low of 34˚. So, that could be pretty nice.
For driving conditions, it is very unlikely that you would have problems with snow. Going there is Sept. would, of course, be nicer but Oct. should also be fine.