Ken,
I would say no, you did not make a mistake. Most on here know that I have a 6.0 and like it very much!!!!! Mine is not in a work truck, but it my "family car". If it has been maintained well, you should not have a problem. Keeping the service up on a 6.0 it the MORE IMPORTANT than on alot of other engines. Change the Oil and Filter at 5,000 miles, no more, change the fuel filters at 10,000 miles, no more PERIOD!!! Use only Motorcraft Filters. Some did not have the water block membrane on the filter that is in the FICM on the frame rail. Use good oil..... I have been using the Motorcraft brand oil, and it seems to like it better than Rotella. Use ONLY diesel in a 6.0..... NO COCKTAILS.... you WILL kill injectors. Also, do not run a 6.0 out of fuel, if you think you are going to run out..... SHUT OFF THE TRUCK AND WALK.... the fuel in the injectors acts as a pad for the intensifier piston and the "pintle". No fuel, it will damage injectors. On the later 6.0 when the fuel gets very low, the FICM/PCM will cause the truck to idle rough.... to make you look at the gauges and see what it going on.... if it starts to do this.... feed it ASAP!.
I've had my truck for a little over 4 years, and the only things I've had to do to it is:
Routine Maintenance (Oil and Filter Changes)
Fuel
Brakes
Tires
Water Pump Weeps when the ambitent temp is cool - on the to-do list
Shocks
Front track bar link bushings are shot - on the to-do list
It is 100% stock and 100% factory. I have the factory EGR Valve and Cooler, Factory Oil Cooler, Factory Turbo and head bolts. Even still have the cat on it....
In my opinion also. NO TUNERS!!!!!! A 6.0 will do everything I ask of it, in stock trim. It has plenty of power and I still get decent economy. Mine is in and Excursion, and I have 3.73's. I get ~16 in combo commute, and ~20 on the highway. I keep it at 2200 or less (cruising) and that is the best economy. I am sure in a 550, you have 4.53 or taller... so 2200 would be crawling..... Keep it maintained, it should stay happy, and keep you happy.
Also..... one other thing to keep in mind..... DO NOT BABY IT.... that will make it very mad..... they like to work.... don't beat it to death.... but don't be afraid to wind it out every once in a while..... I have driven mine like that since it bought it.
Another thing, it is not an IDI.... a 6.0 will wind out father.... a 6.0 has a 4000+ rev limiter, and it is not uncommon on a brisk accel... not on the floor, but hot footing it, for it to shift at 3500+..... I have made the comment before that most IDI'ers would have a heart attack at the revs a 6.0 runs......