I think most here have said what to watch out for and they are right.
I just bought a little larger 5er (12,120lbs dry) and in all reality it was to big for my 2006 F350 SRW WITH a 6.0 diesel.
I could have beefed up the suspension added bigger brakes and an exaust brake and then put on a sway package and tuner, but then all these aftermarket parts would have been needed to be installed and I was looking at about $15,000.
Not my idea of a good time. Even though these parts are good I very much doubt they would last as long as a factory install with everything tested together like a factory truck has been tested.
I went with an F450 instead.
Take the time to be safe instead of sorry and look for minumum of a 1 ton dual rear wheel truck. A gasser will be worthless IMHO, and I am sure it will be even harder to find a used gasser with a 4.30 rearend because people who bought them wanted better gas mileage so most opted for 3.73 rearends.
Sorry for the downer opinion.