Hello to all

I just joined this forum as it looks to be one of the best for Fords. I am giving up a jeep for a Ford Diesel Truck and was looking for your thoughts and advice. I'm leaning toward a 99 to 03 f350 or f250 with preference being to the 99-00 f350, but I've seen a couple newer 6.0 that have been rebuilt (oil pump/head gaskets and studs/injectors/so on). Here are my parameters:
Diesel Crew Cab, short box, will be loaded with 250 to 1000lbs of fun most of the time it is driven. 7.5k is my average use per year (I have a company truck). I want to get 20mpg on average and be in the 300 to 400hp range.
When I'm done with it (I may find one with the upgrades or do them myself) it will have oil & tranny coolers, fuel filter/separator, exhaust, turbo, intake, tuner and gauges (egt, tranny temp, boost), preheat delete, fuel cross over line, and possible new injectors. A 6" lift and slide out bed will also be done.
I want to have this truck for the next 10+ years, so I'm trying to get a feel (as 7.3 owners) from you, given this list, if I should be looking at a 6.0. Another important question: if the truck has been well cared for and the price is right any reason to stay away from a 150-200k mile truck? I've seen several in this range of miles, many with upgrades already done. I've found several with 100k, but they usually have many less upgrades. The pricing is about the same 15k or so (but I would have to do many things on my list to have a lower milage base truck).
Anyhoo, I realize this is a long post, but thanks in advance for your ideas.
Garth