Not sure of reasoning to go backward in years...except to get a 350 dually? I had an '07 350 dually that I put 98k+ on when the egr coolers went bad. Ford fixed it but I went ahead and traded for my '09 450.
The truck will handle the trailer just fine, but I think you need to be ready to work on the engine some--basic problem is the EGR/oil coolers/head stud stretch. If you are in a non-emissions state, I would either pull the coolers or get Bulletproof units and replace the head studs with ARP studs. Then use a tuner to help performance.
Many, many people feel the 6.0L is a good engine if the emissions junk is fixed/taken off, but you can't/shouldn't use a tuner without putting the ARP head studs in.
I also had an '04 with the 6.0 and it basically shelled itself out at 125k miles...maybe I'm just too hard on them.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired