ISC, as is it's slightly larger brother the ISL, are solid Medium Sized diesel engines.
The ISL in CC's, as well as the CAT C9 (Another fine Medium Sized diesel.), do have Compression Jake Brakes - which is IMO an advantage over the ISC's with Exhaust. (Note, not all ISL's have Jake Compression. Newmar may, or may not, have it on the ISL, depending upon year and specific Model you're looking at.).
Both ISC/ISL, will run well for you. Exhaust Manifold warping and or cracking can be 'if' not 'when'. And older CAPS era engines had Lift Pumps that can sometimes need replacement. Neither of these are a given 'going to happen', and IMO are not a deal breaker. Just know it going in, that it's something to keep an eye on.
If you end up going with a coach with the ISL, and it is in or near the 2006 model years - get the specific engine in questions Serial Number, and check this forums Cummins section, you'll find a Stickey with the serial number ranges of potential Wrist Pin problems. Another item to be aware of, and if the engine is suspect, negotiate a lower price point for the risk of failure... Enough to cove a Extended Warranty, to protect you.
07 Country Coach Magna Rembrandt 45' ISX600
Roo II was our 04 Country Coach Allure 40'
OnDRoad for The JRNY! Enjoy life...