Re the gearing: you'll want more choices than that and you'll want it field serviceable, I'm pretty sure. There are 15% grades on this ride. That means don't get another ring, they're a pain to install, go for cogs. That's my opinion. But you've got some time to figure it out. Putting in some hilly miles will help you figure it out.

The Turbo: I love 'em. I think they're not made anymore just because they're not flashy enough and they're kind of heavy. Again, put in some long miles on it and see how your ass feels.

Speaking of your ass, yeah, ditch the underwear under the shorts. The point of padded shorts is not to give you a soft place to sit. Cotton chafes when it gets wet and leads to saddle sores given enough time. The spandex and chamois in your padded shorts don't. For them to work properly, they need to be the thing that's in contact with your skin. If you go with chamois cream, it should be applied liberally before you develop sores. They say it's not necessary with modern synthetic chamois but some people swear by it nonetheless.

The best way to figure this all out is to stop sitting on the computer and get out and ride. A lot. There are training schedules online that will help you increase your mileage.