I'm not familiar enough with the 450's and up class, so I don't know if they got different parts than the 350 and 250 or not. However, from what I have read, most everything is the same...though I have heard the emissions equipment was different. I just don't know how. I don't want to dismiss your questions, but a lot of what you're wanting to know can be found with a quick search. Probably look for the term bulletproof or new truck, and in those posts you'll find what you're looking for. However, just to cover some of the items.......
Using a OBD-II adapter (bluetooth for Android and wifi for iPhone...don't skimp, get a good one) and an app like TorquePro or FORScan, check for codes and look them up. Yes, you have the scangauge, but it misses a bunch of stuff that FORScan catches. That should begin to tell you a story. Next, check the coolant and oil temps when the engine has been sitting for a good 8-10 hours. They should be identical, or a degree or two off, but no more than that.....and they should pretty closely match what the outside temp is. Once you know those are good, check your oil temp deltas when the truck is fully warmed up and driving 55-60 MPH. Your gearing might throw this off a little, because a higher revving engine will give higher oil temps, but you shouldn't see more than a 15 degree difference. If so, you may have a clogged oil cooler. If you do, you'll want to do a reverse cooling system flush with Restore followed by an oil cooler replacement. After that, go with an EC-1 rated ELC coolant. Oh yeah, and a coolant filter.
Another thing, you have the studs, so that's good, but you don't know if the STC fitting, dummy plugs, or stand pipes were updated. I'd ask the previous owner to see if they know. If they got work done at a garage, that garage might keep vehicle histories and be able to tell you what work was done. How old are the injectors? Do they have stiction? If so, try a lighter, full synthetic oil like Rotella T-6 5W-30. If that doesn't work, or if the stiction is exceptionally bad, get some Archoil 9100 and pour it in. That will take care of all but the worst injectors. Ummmm.....electrical is key with these trucks. If the batteries are 4-5 years old you will likely want to get new ones. I recommend AGM style. Also, with one of the apps, check your FICM
voltage both running and when cranking. It should never go below 45 volts. If it does, it's time for a new one. Best bet is having it repaired by FICMrepair.com. I also like to get an HO alternator to make sure things charge well. What else? hmmm.....how about clean the turbo. The vanes stick if it's full of carbon, so a cleaning might be in order. Get rid of any fancy filters like K&N. Factory is more than enough airflow for even a modified motor, and it filters best.
I'm sure there are things I've missed, but this is a good starting list. Other than that, I would drain and refill all fluids with a known, preferred product, lube up the u-joints and steering knuckles, etc, and just go over everything and tend to her needs. A plow truck usually has a pretty hard life, so she's going to need some TLC for a bit until you can get a good feel for her.