#1 it is a complete fallacy that motorhome diesel engines last 500k miles - those mileages are reserved for OTR truckers who almost never switch their engines off. Motorhome engines are abused all of the time by stop and go and have similar life expectancy to a gas engine.
A new ISC engine would be around $20k plus installation - his estimate was on the high side.
You also need to find out what happened to the turbo - if the bearing seized then a replacement turbo may be all you need - if the blades shattered then a new engine is in order as the engine would have ingested metal fragments.
You could probably get the old engine rebuilt for $10-15k but looking around for a good used one might be another options.