The single biggest thing you need to do in you planning is work in a ton of "personal" time, where the group can break up and people can do their own thing.

Having 7 people crammed into a single vehicle for 2 weeks is a challenge, trying to do it for 2 months will be a huge test of your friendships, and honestly, if you all come out still being friends at the end it will be quite impressive.

I know, you're the best friends in the world and nothing would ever change that (or insert similar story that is always said before a trip like this), but roadtrips are far more stressful on a friendship than even living together, and there is simply no way that all of you will agree all the time on what to do an what to see. Building in lots of time for each of you to go your own way is really essential for this trip to be a true success.