Unless you know diesel engines or really think you want to GET to know them then steer clear of them. They are not plug-n-play like gasoline engines.
"A little rust" means a lot of different things to different people. Where are you from (originally)? I ask because to people from the southern half of the US a little rust means there's a spot on the trailer hitch and to those from the North or northeast it may mean that only two wheels have rusted off.
It's a beautiful rig for sure, from what you've shown.
Looks like it has:
1. Propane because it has a stove and a hot water heater, so hopefully it has a furnace as well?
2. Does it have AC in the camper part (Starcool or the like)? I do not mean dash air conditioning.
3. It's a '50', or RB50. This is Sportsmobile's number for that floor plan. It is the arguably the most sought after and popular floorplan. Huge plus in my opinion.
4. It has the small Norcold fridge which can be noisy and slow to cool and aren't very big at all but it's normal for that van. You might like an upgrade sooner than later.
5. It has CRL awning style windows. This means they pop out from the bottom and can be opened for ventilation in the rain. These are good. Many SMB's have side sliders that cannot be opened in the rain.
6. It has the nicer Fiamma awning. Thi$ i$ a nice awning.
I'd say $40-45k because of it's age. It's no different than a mid 2000's model but even with the same mileage that would bring 10k more. If they have it listed for much less than this buy it quick or let me know where it is.