Nothing wrong with a CAT. Even though they have left the on-the-road engine market in recent years, they make good stuff. I had a CAT in my previous coach and it ran like a top.
More important is to make sure it has been properly maintained, but that is true with the transmission and all of the coach systems. You can insist that the seller let you pull an engine oil and transmission oil sample and send it for analysis. That will tell you a lot about the overall health of the drive gear. If there is a CAT dealer/service center in your area, they may have their own oil lab. If not, they can certainly provide the sample bottles and tell you where they send their samples.
The Essex is a great coach. We looked hard at a 2005 when we were last shopping for a coach.
You can also post your question in the Newmar owners area. Plenty of Essex owners there.
2007 Country Coach Allure Siskiyou Summit, sold/traded Nov. 2018.
2019 Grand Designs 384GK-R 5th wheel. Glen Allen, VA