Good one, Eric. I've done the same trip 5-6 times, also by car, at different times of the year. First time, 1967 (when I was about the age you look to be in your photo), last time 2013. A delight to the senses, each and every time.
APUG member Frank has traveled extensively in the USA. I believe he has posted a lengthy and detailed story on his web site about one of his motorcycle trips, I read it and greatly enjoyed it some time ago. Very inspiring. Do look it up.
September is a particularly good time to go. After Labor Day, no crowds of holiday drivers clogging up the highways. Gorgeous weather. Warm days, cool nights. You will likely miss the glorious beauty of the trees changing into their autumn colors (or maybe at the end of your time on the road).
Fifteen days is pushing it a bit. Try not to spend your time driving, driving, driving. Avoid shooting your landscapes from the driver's seat. Get out of the parking lots and walk, walk, walk. The best views are usually about 10-15 minutes away from where you left the car.
The SUV will bankrupt you for gas. Rent the smallest sedan you can find. Plan to stop driving before 5 PM every day, hit the country towns off the main roads, find a good not too expensive motel, they do exist. You can eat well from the supermarkets. Lots of salads, fresh food, cooked chicken,whole wheat bread. Wash all that down with good California wine (in moderation of course).
Also buy a good road map and read it over your lunches and dinners. Pinpoint places of interest. Google them. Plan to visit them. It's the best way to go. How many times have I wished we had the internet and Google in when I first made this trip in the long ago Sixties.
Is film cheaper in Japan? Maybe, but I doubt it. Buy your films in California. OL services will ship to your friends in San Diego. Have it waiting, nice and fresh, when you get there.
The Leica (and which lenses?) is a good choice. As for the Instax, are you taking it with a specific plan in mind (= exhibition)? I personally would pass on the Instax and buy, borrow or beg (don't steal!) a MF camera like a Fuji GA or GS, and use it with black-and-white film. A yellow or light green filter will give you wonderful negatives with exquisite detail and tones. Remember to compensate on exposure for the filter factor. Your call (choice of cameras) entirely, of course.
I'm almost 70 but I intend to do this same trip at least one more time, before I move on to processing my films in The Big Darkroom In The Sky. Also the drive from SF to New Mexico, another life-enhancing trip. I've done this one 13-14 times, and I still want more of it.
Be sure to post images for us all (and especially me) to enjoy.