A lot depends on how much you want to carry. I like to pack very light and would be happy on a CF road bike. I rode the Southern tier with a guy who used a CF road bike and a trailer and have met a number of others doing the same on the Trans America and on the Pacific Coast.

As far as lights go... Most of the folks I have met on tour carried minimal lights and seldom use them. Carrying a minimal blinkie for those rare times you get caught out is sufficient IMO.

Saddle comfort... In my experience it is more about getting used to what you have than in getting the right saddle.

Training... Spending lots of time in the saddle helps, but on a coast to coast tour you have time to train as you go. Just don't over do in the beginning if you take that approach.

Edit:
I should have mentioned that if you do go with the road bike, you will probably need much lower gearing than is standard on it.