Great choice I love my Cummins Aisin. Good choice going with a DRW as your camper selection is limitless.
I agree with the Aisin and you can't go wrong with4:10s but I posted regarding gearing and I think for just carrying a TC 3:42s would work. How heavy is you're 5th wheel?
I would probably "error" on the side of 4:10.
Where are you buying? Make sure you get at least $10k off sticker for a Tradesman up to $14k or so for Laramie or Longhorn.
Definitely get the 5th wheel prep. Well worth the $400 and check out the BW 3600 hitch.
Not sure about the air system but I think it essential a Firestone Airbag set up. Different than the 2500s but I think it works well.
Your Torklift won't fit the new style. They are all different, even my 16 LB to 17 SB were drastically different.
I have a brand new set of front TL if you're interested.
Having both the Bighorn and Tradesman, I would spend the extra few thousand dollars for a Bighorn or Laramie. A lot of " nice" features.
Enjoy, you won't be disappointed.