A 16K will work but as said an 18 or 20, while overkill, will serve you better when you upgrade.
Slider? Most, if not all of the newest front cap styles allow pretty tight maneuvering without a slider. That said, if your bed is anything less than 6-1/2 feet, a slider should be used. Sliders have more moving parts that will allow front to rear movement in the hitch assembly that you will really notice in the truck. It's called chucking. There are several brands of trailer pins that reduce this chucking. Trail Air Tri Glide, 5th Airborn and MorRyde are the ones that come to mind.
That dry weight of 12,300 will probably be up around 13,000 at delivery. It seems they never get that "dry" number right.