Your questions are good. Look at all the wonderful conversation you started!
I would start training whenever it feels good to get out on the bike. Winter time, while you’re not likely to ride, is a good time to get the bike tuned up.
In 2014, I started riding short rides in April, building up to longer rides through May with a handful of 50-mile rides in June and July. I had 500 miles total before doing RAGBRAI, and I made it the whole way.
In 2015, I found that I was enjoying riding more, so I did a lot more training rides, and several of them were longer (I aimed for 100KM rides, which is about 62.5 miles, give or take). I had 1000 miles by the time I did RAGBRAI and had a much better time.
I chose my charter based entirely on the fact that it was the one my college alumni office uses. They have a “RAGBRAI in Style” option that puts you in a hotel room each night: Bikes to You
All of the charter services, as well as the other options, have their pros and cons. My experience with Bikes to You is that they’re smaller, the group is very friendly, and there’s good personalized service. One down side is that Kyle never seems to know how to find the best showers. They also provide shuttle service to/from the Des Moines airport.
Too many options.
Rule 1: Have fun!