We have owned two coaches with the Cummins ISX 525hp engine. Both have been exceptional.
How many miles are on the coach?
An EGR system delete may be an option. Some may disagree, but it has worked well for others.
So far the ISX 525 engine has been reliable. I don't think Cummins even offers an updated cylinder head for this engine.
I installed the Ugly Fix on both of ours and that has worked well. It tells the engine ECU that the coach is above 7000' and locks out the EGR.
https://www.theuglyfix.com/Cummins-I...duct-s/123.htm
The big problem engines are the 2008-2010 ISX 650hp twin cam versions. Some folks have had very expensive issues caused by the EGR system recirculating exhaust gases containing acid and the acid has compromised the #6 intake valve. An RV owner with this engine should consider the new head replacement and/or a complete EGR delete. Plan on spending $15-25k on the new head and associated parts and labor.
The ISX 600hp engine may get by with just the Ugly Fix or possibly consider an EGR system delete.
I would personally purchase a coach with either the 525 or 600hp ISX engine. I would hesitate purchasing a coach with the 650hp ISX unless I could budget $25k for the updated cylinder head with Inconel stainless valves and special seats.
Hope this helps.