So... what are your short list recommendations for picking a used ambo these days?
It depends on many things.
1) how much will you drive it? People pay high premiums for diesels but unless you rack up a lot of miles or are towing then the fuel economy will likely not make up for the increased costs of maintenance.
I worked out my fuel costs, repairs and maintenance vs what a gas engined ambo would have cost me and over 4 years / 10,000 miles per year I would have been money ahead with a gas engine. If I'd had a full engine failure then I would have been really hating life as a professionally rebuilt and installed 6.0 is more money than I paid for my entire ambo. A rebuilt Vortec or Ford 5.4 or V10 are way more common and less expensive.
2) Do you need/want 4x4? If so, how big of tires do want? Converting the van based units to 4x4 is pricey. Lifting it enough to run 35's or bigger is pricey. UJoint 4x4 conversions are roughly $30k. It can certainly be done for less but only if you're doing the work yourself and possibly cutting some corners. With the F series based it's no big deal to run 37's or bigger. the Kodiak can run 42's without even needing a lift kit.
3) how much of the maintenance and repairs will you do yourself? A lot of people report that's it's challenging to find a shop that will work on a diesel econoline.
So, if you need 4x4, plan to tow and want 4x4 then I'd go with a Type1 (pickup based) or the GM Kodiak.
If you don't need 4x4, want a bigger pass through, and shorter overall length then any Type 3 cab and chassis is roughly the same but stick with a gas engine unless you a) need the diesel or b) are prepared for the extra costs.
Of course if you want to be impractical because it's fun then buy whatever you like, just be prepared for the expenses. When mine isn't in the shop it's a ton of fun and despite the costs I am planning a 4x4 conversion but if I was doing it all over again with what I know now I would have chosen differently.
Hope that's useful to you!