David,
Sorry it took me so long to respond to this thread. I've had both an '01 with the 7.3L powerstroke and the '06 with the 6.0L powerstroke. Both were 4x4 Crew cab Duallies with the 6-spd manual trans and 4.10 gears. The cam sensor failed on the '01 and the turbo went on the '06 (both were covered under warranty). Other than that, I did not have a lick of trouble out of either. I exceeded 120,000 on each before letting them go. And, I only sold them due to pending deployments. Otherwise, I would still have one or the other.
I prefer the duallie, but a SRW crew cab will handle your tractor and horses. Anything heavier...well, you have seen what I have in the backyard to take care of any heavy hauling. They are available when needed.
What can you legally tow? Not sure about a SRW, but I have gone across the scales there in Lorton at 29.5k with the '01 and 29.4k with the '06 (got the pictures to prove it too

) . I got the green light both times. Both trucks pulled those loads up I-95 with the cruise set at 65. I might have had to drop back to 5th once or twice. So, I reckon you are good to at least 17-18k. Just watch your speed and increase your stopping distance.
Check all the hoses coming off the turbo and that are part of the intercooler. I didn't have any problems with the 7.3 but I kept blowing the hoses off the 6.0 with minimal boost. If you see any traces of oil (fresh or otherwise) on any of the hoses replace them. No damage is done when they blow. But, it sounds as if a rocket is shot off underneath the hood and all power is lost. It took me a while to figure that out and it was frustrating as all get out.
Bottomline, I'd recommend an '01 and earlier, or an '06 and newer. Looking forward to sitting under the shade tree and spending the rest of the afternoon checking it out :thumbsup:
Hawk