Hard to miss Mtns on "northern route"
John, Sounds like a great road trip adventure. In your posting you referenced "...leaning toward the northern route. I would like to avoid as many mountain passes as possible..." Unless you go to the Artic it is pretty difficult to get over the Rockies and Cascades without going over passes.
October is just about the perfect month for road trips in North America. Although there is a chance for early snowfall -- generally the days are cool, the nights crisp and reasonably dry roads are the norm.
South Dakota is pretty amazing place -- were you considering taking I-90? The least mountain driving route would be I-10 and then up the coast (but you will still have mountain passes on this route). What is your concern about the moutain driving? Take it slow, use the turn-outs and enjoy the views -- it is going to be great.
Mark