With my experience with electric fans you’ll want the fans to run all the time to pull air through the intercooler, not sure if yours have a high and low setting. But I wouldn’t have them off ever.
Not sure about Lbz and older but on my lmm I got an alternator from an L5P they’re 280 amps, if you need more get one from an L5P ambulance I heard they’re over 400 amps
I would just run the standard 16.5:1 and call it a day. Run higher rail pressure if you need to. The difference between 16.5, 16.8, 17 you won’t notice a difference
I install a small solar panel and charger to all my trailers so they’re ready to go at all times. And just use the factory connection as extra. But since you already have large wiring at the back of your truck that would be best.