My old 06 5.9cummins just got out of the shop from being worked to death yet again and I have decided to get a larger rig. I have looked at a IH with a dt466 and a 5+2 and a freightliner with a 8.3 Cummins and a Allison auto. I pull goosenecks loaded to the gills with hay and I want to make sure I have the right truck for the buttwhipping its about to receive. I don't have a lot of knowledge in the medium duty class of trucks and need all the help I can get.

 

just about anything new is going to have issues. i will say something like a kw t300 is probably the easiest thing to work on.

 

466 is a good engine. Look for one with a different transmission though. I hate the Spicer 5 speed. Coupled with the 2 speed rear end you have 3 gears you can really split, but 4hi and 5low will be so close you effectively don't have any ratio split between them. Works pretty good with a v8 but not so much with a diesel.
The 4700 and 4900 will have a lot more room inside than a 1700 or 1900.
They pretty much all use the same running gear, just get different numbers printed on the door sticker, so you have heavy components even if your GVWR isn't. You shouldn't be able to load one heavy enough with a gooseneck to hurt it. But they won't go where a pick-up will due to the increased footprint and stiffer suspension.