You are right - assuming we beat Buffalo (which is not a sure thing at all), NU is going to have to beat 4 Big Ten teams. Where are the 4 wins?
I'm writing off OSU as a loss now. Same with OU (even though that is a non-con). It is reasonable to think NU might (MIGHT) be able to beat Purdue and Northwestern at home. That's 2. I suppose NU will have a shot at MSU and Minnesota - though I suspect NU will be an underdog in both. You certainly cannot pencil in either of those games as a win.
That leaves us with home games against Michigan and Iowa, and a road game at Wisconsin. I think NU is going to have to win at least 1 of those games in order to get to 6 wins. They may need to win 2, or even all 3.
I just don't see it. I have seen nothing from Scott Frost's teams in 3+ seasons that indicates they can rise to the occasion often enough to get to 6 wins. In fact, I think we are going to end up with 5, gnashing our teeth at giving away that Illinois game.