Great replies, thank you so much!
Alas, I wish I could get the service records. The school districts don't pass them along usually. The bus isn't marked, but maybe I can find out which district it came from and ask them directly. Not likely to get any info in time to make a decision, but might help down the road.
I am bringing my ODB meter, and I hope the connector will be right, it's an automotive meter and is somewhat older. An Innova 3020b.
Thanks for mentioning the CARB restriction on diesels. I just did some reading and yikes. Based on this useful chart
I just found, the type A Collins 20-passenger should be under 14001 but its still a wrinkle I hadn't considered. (BTW, yes, the body is the one with the flat roof and straighter sides.)
One consideration I have is total cost of maintenance and ownership. The starting price is pretty much the same, but what will the bus cost to maintain? What kind of servicing regimen keeps diesels happy, and what does a typical service cost? I have a rough idea for gas engines, but again the diesels are outside my experience.
Before I retire I doubt I'll put as much as 5k miles/year on it, for the next 5 years or so. After I retire and can travel more, another 10+ years with still under 10K miles/year. 15 - 20 years out, who knows if I'll be driving it or will need to hang up the keys? So either engine should have the service life I'll need.