The devil is in the details... If the ISX is EGR only (Usually CM870 build), these engines are not as potentially problematic as the later years ISX's with EGR + DPF (Usually a CM871 build).
A 2007 coach, could have either engine, depending upon how early in the Model Year the coach was produced.
The CM871's ISX's were the ones you could have read about that had the 'Dropped #6' valve. Due to the soot/gook build up in the Exhaust/EGR/DPF plumbing - working it's way down and dripping onto the #6 (And sometimes #5) Valves. A corrosive effect kicks in, and can result in the valve breaking down and taking out the cylinder - and more. Cummin's did come out with a new and improved cylinder head with inconnel alloy's, that handles the soot/gook drip.
Seems like the addition of DPF in the CM871 build, resulted in the block being installed at a slight angle leaning towards the #6, and that is why it is more of a chance of having problems.
Usually the ISX600 and 650 are the ones you read about with the #6 Dropped Valve issue. But some ISX600's and 525's with CM870 builds, have also reportedly had dropped valves too. (And this is from memory, as I believe a Monaco owner posted in this section about an ISX525, which I believe had EGR only, had Dropped it's Valve... BUT FOR SURE DO SOME SEARCHES on that, as I'm going from memory 3-4 years ago. I've never personally met or communicated with an owner of an ISX600 EGR CM870 engine that lost a valve.
And, on the Over The Road Truckers forums, Rawze's is one such site, the ISX did not seem to have as many reports of Valve's Dropping. The thought was because they were putting lots of miles on their engines, vs an RV'er that may due 10-15K in a busy year.
The 'Smog Plumbing' of both the EGR and DPF were basically counter productive... Pumping Engine Exhaust back into an engine, not too swift of an idea. As the EGR's start having problems, the engines run dirtier and dirtier, lower MPG too - thus the counter productive comment.
Me? I made a decision to go with an ISX600 EGR only CM870 build engine, in our current coach. I had planned to go ahead and have Cummins swap out the head's with the new and improved head, at time of purchase. But instead, put about 65% of the cost of doing so, into as good as possible Extended Warranty that covers the full coach, and chassis. My point, is I went into the purchase with open eyes, and feel I've established a reasonable counter measure to the chance of an engine Dropped Valve event - which from my research, was very low on the ISX600 with CM870 build engine.
Opinions will vary
Good luck on your hunt,