If you are buying a truck, get the most truck you can. I'd rather have too much truck than just enough. On newer trucks the ride difference between 2500hd and 3500hd is not much. SRW or DRW? Depends on preference and if it's a daily driver or not. We prefer SRW for overall use.
Before you buy a slider hitch for a 6.5 bed, look at the sidewinders. Much better solution. I find sliders to be too heavy, take up too much room, and seem to bind when you need them most. If it would fit in my garage, I would prefer a long bed for towing though I find them aesthetically unattractive. Then no need for special hitches.
Enjoy your new rig!!
Scott and Liz - Southern NM
2012 Wildcat Sterling 32RL - w/level up (best option ever)
2007 Chevy 2500HD 4dr short bed Duramax w/allison
Reese Fifth Airborne air ride king pin coupler with Sidewinder