Used 5th wheel &Truck purchase advice needed.

We had a used 1992 lance 24 ft. 5th wheel for past twenty years and pulled it with a 99 Dodge 2500 Cummins Diesel 2 wheel drive. Yes, it looks like I will have to upgrade my truck to a newer one ton 350. It seems the newer 5th wheels in the 27 ft to 32 ft. or more range would take a one ton to pull the unit safely. Don't know if the single rear tire type 350 truck would be OK or it is much better to have a 350 dually?

You sure don't need a DRW for 27'-32' 5th wheel trailer. However trucks don't pull trailers determined by a length but by the trailers gross weight numbers. Look at the trailer dry weight numbers and its gvwr and CCC numbers. They all matter.
As mentioned some of the new gen one ton SRW trucks can handle up to around 15k all depending on wet pin weight numbers/hitch weight/people/and gear in the truck.
Choose the truck with the higher payload numbers.

Believe me, this time around I will get a truck that is heavy duty “camper special” with disc brakes all the way around and a compression brake/Jake brake and or a heavy duty transmission like the GMC Alison to help slow you down going down hills. I don’t know what rear end gears to get.

Compression brake ? Jake brake ? The Jake brake is a trademark name for a internal compression braking inside the engine. Ram/Dodge/Ford/GM doesn't offer one for their diesel engines.
IMO you thinking exhaust brake such as the one Jacobs made for the 5.9 Cummins engine or B-D or Pacbrake and other exhaust brake manufacturers. These types aren't used any more on the new gen diesel engines in our LDT's.
They now use the turbo charger for exhaust braking and is hands down far superior to the old tech exhaust brake we used with our old 5.9 engines.

Auto tranny ?
Ford has the 6R140 Torque-shift....GM has the 1000 Allison ....Ram has the Aisin AS69RC. All three are super tough and made for working.