The narrowest I ran was an Ardent 2.6 wire bead, it’s the same casing and width as an Ardent 2.4, the tire flattened out some, but seemed to ride just fine. I wouldn’t go any narrower as the Ardent sidewall was just as wide as the rim, any narrower and it might not maintain good bead contact with the rim.
A Duro 3" is a better fit, I believe that Muniaddict runs a Large Marge rim and a Duro 3" on his KH 24. Once Summer comes back, I will probably mount a Duro 3" since a 4" tire is a lot of work for distance riding.
Yes, it would ride nice, the rim is a little heavy, but other than that it would be a nice muni, esp if you like the wider foot position. Note that the Ventures on an Oregon hub are about the same width as a standard hub with Moments, so you can get a really wide and stable foot position by changing to Moments. I run Moments on all four of my Oregon hubs; yes, I have four: 26, 29, 32, 36.
The Oregon pictured on Ebay is a first generation with the disc brake mounting tab welded to the frame, and a wire bead Larry 27tpi. My original Oregon had the welded disc brake tab, but Nimbus replaced it with the new Oregon frame and D Brake at no cost. Some of the first generation Oregons also came with the heavier LM DH rim, which does not have the rim machined inner wall.
I’d pay $400 + $50 shipping for the Ebay Oregon, maybe a bit more depending on what “extra tire” he is including.