I spray my underside with clean light oil for rust prevention; so far haven't had any issues in the PNW; unlike many vehicles. My father grew up on the Gulf Coast, and noted oil rig trucks using high dollar undercoatings, would rust out in short order, but most of the old timers who sprayed the bottoms of their rigs never had issues.
The truck in the CL ad is nice! If you get it, perhaps drive it a little bit, and see how it goes; keep some money handy for any issues needing correcting(such as oil cooler if need be). The arm from the clutch master cylinder sucks, and can fall off; so I'd definitely look into upgrading that.
My 7.3 zf is NA, but runs out fine. I'm trying to learn the powerbands on this truck; I drove it through the mountains a couple weeks back with several 7% long grades, it'd bog down when my top speed was only around 65mph, but driving back through the same mountains I went around 70mph, and with that little bit of rpm increase, I guess I was in a powerband, the rig didn't bog at all.