A few comments.
I would definitely get a wdh with sway. The system built into the truck is for active sway control, i.e., it has already started. The hitch sway control attempts to start it from ever happening. As they say, an ounce of prevention is worth a pound of cure.
I would consider a 350/3500 truck if you plan on going to a fifth wheel.
I would get the brand you like the most. I have a Ram 1500(work), F150EB (primary) and Chev 2500/gas (farm) and on the farm, we have ram 1500/hemi 2500/hemi and 3500/cummins, chevy 2500/dmax and 1500/5.3. We get the best deal for what fits the needs at the time and they all work great.
I would prefer a diesel for towing. Yes for all the standard reasons, but one that gets overlooked is for filling up on the road. Hard to get in and out of interstate gas stations. Go to a truck stop however, and you will have a couple of acres to maneuver as long as your are getting diesel fuel.
best of luck
Joe