Don't forget that, when you're gathering specs or asking questions, you should find out what model or series of transmission the prospective vehicle's got. I think the first one with the VT365 will have a good option...either a 2000 or 3000 series. I'm not at all familiar with the VT365, so I'll defer to those in the know. Add asking about the tranny to your to-do list.
The 3126 is a good, reliable engine and probably also has a 2000 or 3000 Allison, but someone will need to check me on that as well.
As to the rust, I'm looking on my phone but it looks like surface rust. Is there flaking or spalling or penetration of the rust?