30 minutes is good, but most of diesels never get to operational temperatures while idling.
I would take all the seller stories with big grain of salt.
When I was buying mentioned DP, the widow said it was park for 5 years.
But stickers expired 7 years before.
If the seller really run the engine for 30 minutes every week, he had to refill the tank several times.
Ask him how did he bring the fuel and that will give a hint how truthful the story is.
As you can see from replies, there is no firm rule what might go bad in the storage.
On quality coach you will need nothing, on cheaper one a lot.
Take 2-3 hr for walking and checking the systems.