Interesting that you found different headroom on two boats of the same vintage....
Anyhow you seem to have the right attitude, and if the Cat 30 is what you are set on at least you have literally hundreds to choose from (probably in California alone)
I was chastised recently for stating that a Cat 36 was underpriced at $35K..... apparently that's book value for those... so that makes the $38K pretty high for a 30. It should be pristine and well updated to justify that price. I suspect the lack of offers is due to the unrealistic tag.
Hard to say why (or if) the engine may have been changed... the universal is generally a good powerplant, quiet and parts are not too pricey. Hopefully you will be able to get that info from the broker or the PO.