I live in Aberdeenshire :-)
Late September and early October is usually good weather-wise I have found, though of course daylight hours are shortening by then. Not so many tourists around and school terms underway so roads are quieter. Most tourist attractions that I know of will close at the end of October, so you should not have any problems finding places to visit.
Aberdeenshire is home to the Castle Trail
and a Whisky Trail is easily accessible too
Not forgetting the Cairngorms National Park
Although you are visiting at a quieter time of year, I would book your accommodation ahead simply because you may have to do this if the six of you want to remain together overnight. I should think you could wing it with regards accommodation if you looked at hotels, but by their nature Bed and Breakfasts are quite small (usually no more than three letting rooms for a maximum of six people) and your group may have to split although guest houses tend to be larger so potentially could accommodate you all.
Having two cars would give you flexibility where an itinerary is concerned because maybe not everyone will want to do the same things on any given day. Also if you do have to stay in separate lodgings two vehicles would be better than one in that situation.
Staying in Aberdeenshire means you are well placed to visit areas such as Loch Ness and Culloden, Aviemore and Kingussie, Pitlochry and Blair Atholl or Dundee and Glamis. I have visited all of them as day trips from my home.
Any ideas about the places you might like to visit, or things you might be interested in doing?