3500SRW is a nice fit for that 5th. I'm in a new-ish 2500 and am amazed at how fast I run out of weight. And mine's a gasser with a 3000# payload. Same truck with a diesel has about 800# less. So, starting from scratch there is no reason to push it. Just get the 3500 and be done with it.
As for dually vs SRW you don't need the dually but it would tow better. I'm pretty firmly in the anti-dually crowd just because of the parking, bank drive thru lanes and things like that. But, those who own them love them so it's really just a personal preference.