480 is a nice number, if you can find out how they have been run that's a bonus (via logs, not the broker's say-so).
On my boat, they are very easy to work on, each resides in its own engine room with stand up head room and plenty of walking around space where maintenance items a re located. The service manuals are still available from DD, I got mine 5 years ago and it had been updated this century, forget the exact year.
Yes they are very noisy when you are next to them inside the boat. Having run boats with Cats and Lehmans and Cummins, the DD is definitely louder at similar RPM. But with the right exhaust set up they sure sound beautiful outside, many love the sound of Detroits more than any other engine, even those that don't otherwise care for them. On my boat, they are not very noisy at all up in the pilot house and quite quiet up on the bridge. In the PH you can converse at normal voice volumes. But having run boats with Cats and Lehmans and Cummins, the DD is definitely louder at similar RPM.
It is essential that you find, through references, a mechanic who knows them to survey these very thoroughly prior to purchase. To do it right can take two days.
Be religious about following the maintenance routine, not so much more so than other diesels, but especially the cooling system, including the intercoolers. You can run them all day at very low speed, but take about 15 minutes at the end of the day and run them within 10% of WOT. Like most systems on a boat, you love them and they'll love you back.
Another very good online source of information about these engines is the Hatteras Owner's Forum, even if it is not a Hatt, visitors are welcome! I will warn you that per my first post, there is a lot of very inaccurate hearsay and folklore about these engines on the internet (shock!). A good, articulate mechanic is invaluable, I have been lucky to find two out here on the east coast and there are a few others.