I prefer an 18 speed as well. You can never have too many gears. I've been driving a 10 speed for a year, it does okay but my truck is super underpowered so I struggle at times when fully loaded. I drive an '06 Sterling, it has a Mercedes in it that is supposedly rated at 400 but everyone that's driven it is convinced it's de-tuned, just doesn't have any balls whatsoever. So I'm not sure I can blame the engine completely, might need to twist the wick on it and she'd be good to go. Detroit 60 Series is a good place to start, N14 Cummins or even the ISX in newer rigs. Hate to say it, but I have a soft spot for a 3406 Cat if you're looking at older rigs. Stay away from bagged trucks for vocational use, they do exist here and there. Nice ride until you blow one up. Walking beam (Hendrickson) is my preference for dump trucks as they will articulate a little more while off road allowing for more tire on the ground while traversing rough ground. However, they ride a little rough unloaded.
Go hard, go fast, or go home