What to consider...I don't know where you are at, so everything may need modified to fit the local situation.  You are looking at licenses, plates, maintenance, on and on.  I could probably buy a good used wagon for the cost of keeping my 3 straight trucks going every year.  Some years a new one.  But I do not feel I could keep up with wagons and we are not nearly as spread out as you are.

On our farm and in my off-farm employment I have driven Chevy, GMC, Ford, Freightliner, and Kenworth trucks.  None of them are without advantages and disadvantages. Most surprising was an F700 Ford with an 18 ... maybe 20 ft bed on propane with an automatic transmission and a tag axle.  Outperformed my expectations greatly

In my opinion if you are going to buy a tandem axle truck get a twin screw (and for the record I have 1 twin screw and 2 w/tag axles.)  I would suggest getting an automatic transmission or if not then get a 6 speed. 5x2's are common but trust me, a 2 speed axle is just another maintenance hassle that can easily be avoided.

And before someone says it, I know, "anyone can learn to drive a Road Ranger".  Same with a 2-speed rear axle.  But considering your apparent lack of experience I'd get an auto.  And speaking as someone who learned to shift a 2 speed long before he was legal to drive, take my word for it a 2-speed is a needless hassle.

I would also suggest air brakes.  And make sure the truck is new enough to have the spring loaded brake cans on it.  It is really nice to back up to an auger, pull the air brake button, and know it isn't moving.  I bought an old 10 wheeler at a cousin's farm close out sale (another long story) this winter that had air brakes but with a cable operated parking brake.  On the way home I stopped by Ralph's and told him to order the parts necessary to convert it.

Diesel, gasoline, propane ... all have advantages and disadvantages. For you I would lean away from a diesel.  There are advantages to each.

I realize I have taken a lot of time to not offer a specific recommendation. That is because it has to fit your situation.  For me a 400 bushel single axle is not enough capacity any more, but I am fighting the urge to go to a semi because it just does not fit our operation.  The idea of pulling a wagon behind a single axle truck was considered, but unless you are dumping into a pit it's not a good configuration. Plus that opens up a whole new realm of legal hassles in some states.

Look at what your neighbors are doing and why and ponder on how it will fit your farm.  And expect not to be satisfied because once you do it you'll find a better way.