My truck is just that (FL60, Cummins, 6-speed). As far as maintenance, I have had no issues. I picked the cummins on purpose though. The CAT option was very loud. The FL series don't have much sound insulation. If I can ever get time, I'm going to add noise protection to the cab. The 6-speed is fine, but I would have liked 7. The sevens are kinda hard to find. One big point on the Freightliner MDT is the long front springs. This gives them more travel and a better ride than some of the other MDTs.
The key to any truck is to have a mechanic lined up. My mechanic is so much cheaper (and more reliable) than the local dealer.
Doc Weaver '98 Freightliner FL60 pseudo toy hauler