I am building a second truck to compliment the big truck right now.
2005 Dodge 3500 quadcab dually, going to remove the box, put a flat deck, 325g of water with a single wand heated setup. Truck weighs 6700 lbs as it sits right now, total gvr is 12,200 lbs.
I was looking for a truck that do the small jobs and be able to kick around town for the errands. I hate trailer setups due to the length and trying to get into some construction sites is not easy either. My big truck which does most of my work is great, just the fuel economy and paper work that goes with it for a 30 minute job makes it impractical. I do agree with getting a truck with a high gvw then you need. My big truck is way over specd, this truck is more for getting to the big truck, and for those small jobs that pop up. Over spec the truck and maintenance items like brakes, suspension, even transmissions don't seem to wear out very quick. Under spec the truck and the savings in fuel, if any will easily be eaten up in the other repairs and down time. Pick your poison, but I always found it easier to do it right the first time then doing it twice
My first truck was under spec'd, 05 isuzu npr with a 14 ft box. Just the water tank, and 800 lbs of machine + hose + fuel put me up at 10k lbs before I put water in the tank. My tank was 600 g and of course the truck was only a 12k gvw, so yeah I was over weight. Only problem I had was tires would wear a little quicker, but nothing else had any problems luckily for me.
Just my 2 cents, you are on the right track watching your weight, too many people build the truck then look at the weight once its done.