Recommended Truck Modifications/Addons

Put the cart in front of the horse. Select or buy the trailer first. Match a tow vehicle to the trailer. A matched TV won’t need modifications or addons.

Buying a new truck, decide what you want to tow first, then choose a truck built to handle your plans.
Tranny cooler, get the factory tow package and don't worry about it.
Pre Luber, not really needed, not sure what type of wear you're anticipating, the factory drive trains are warranted for 100k miles, and I would worry that that mod may impact the warranty, Be cautious at the least.

Buy a truck designed to haul at least 20%, preferably 25% or more in excess of what you plan to buy as a first trailer. Chances are you will realize you want something different in a year or two as you develop your own personal travel style. Odds are, you will grow, but even if you don't, very few people complain about having too much truck.

You might get some seat covers and floor mats.

Save your money on 1 & 2 and put it toward buying the 2500, fuel, and go have fun. No need for either 1 or 2 in a modern truck.