DD15 - Freightliner

Hi All, I just bought 2 x Freightliner argosy in Australia with DD15's, both trucks have little known service history.

1st - 2017 with 685,000 miles
2nd - 2013 with 295,000 miles

I very much doubt any major work has been done to both trucks, the little information I got suggested nothing major has been done.

Is there anything I should have done to them other than a full service? I was reading online that maybe I should do the oil pick up o rings?

How often do you need to change the fuel filters on a DD15, I am only used to spin on filters, not sure how often you should do these type of filters?

The 2013 has a bit of up and down revs at idol, I think they call it loping? Reading on the forum this should be the quantity control valve?

Any other filters that may have been missed if the previous owners was doing basic servicing (coolant, blowby filters etc???)

I have never owned a freightliner or detroit before, any advice is appreciated