Cant help you out on any negotiating tips. I got hardly anything off mine but they gave me what I paid for my 2013 with 15,000 miles on it. It felt so good???????
It's often said that the only difference is a spring or two between a 2500 and a 3500. Well, they must be some very expensive springs. I have not researched this but I dont believe it. My truck is new and I really haven't been under it much. Both my sons have 2500's and I have a 3500. One son looked under mine and said everything is different. I think he even said something about the front drive axle input being different. These are all Dodge 2012 to 2014 trucks. The 3500 axles are HUGE.
Someone was just on here the other day about having bought a 2500 on salesman's recommendation even when he explained that he had a large 5er. I would check into this much deeper before I would go to a 2500.
When I shopped I asked my son what he would do different and he said he would get a 3500 and be covered. It worked out well as I couldn't stomach buying a $35,000 new rig that you couldn't even close the door on. Ended up with an older excellent shape heavier rig. No need to be doing springs and bags etc with a 3500. More ability to haul TC also.