NJJoe wrote:at what point do you need or is a dually recommended?
The text-book answer is when the load on the rear axle exceeds (or approaches) the weight rating of a single-rear-wheel axle. Those that have pulled trailers with a lot of tongue weight and/or a heavy load in the bed will tell you that a dually is more stable thanks to reduced sidewall flexing. On the other hand, dual rear wheels don't run in the same track as the front tires...and that makes for some "squirrely" driving in slush/snow with no load in the back. If you are going to some occasionally towing that is within the capability of a SRW truck, I suggest you pass on the dually trucks.
If you want a diesel, I would try and find an older truck that is in good shape. Out-of-warranty repairs on the current emission-tuned diesels are not for the faint of heart. Particulate filters, excessive fuel in the engine oil, high levels of soot in the oil, poor fuel economy...no thanks.
I have a '99 F350, reg. cab, single rear wheels, and a V10 gas engine. It pulls a 10k boat & trailer with no problem, and would probably do it just as easily with a ton of coal on the back.