Looking for a used truck for a TC - What to look out for?

I have spent a fair amount of time looking into this issue. I plan to go diesel and have settled on some version of a Duramax. Ford diesels have too many issues, the Cummins diesel is great but rests in a Chrysler truck. If I was going gas, it would be Ford or Chevy, essentially even. I have not driven it yet, but everyone tells me that the 3500 rides the same as the 2500. Everyone also tells me to err toward more truck, ie prefer 1 ton to 3/4.