buy a ScanGauge II to take with you on test drives. You're going to need it or something like it anyway after the truck is yours, this isn't a waste of money. Plug it into your car to power it up, and then you can program in the XGauges you'll need.
You do have a bunch of reading to do, but you shouldn't let that scare you away. With some money and work (how much, depends on the current condition, which is what the gauge is for) it can be the best truck you could ever want. Lucky for you, you've asked the question before you bought the truck. We will be able to help you know what to avoid, and what you might just use to get the price down.
Any vehicle you were interested in, you'd get the CarFax, of course. With Ford vehicles you also want the OASYS report. This will tell you all the work done under warranty. Good info. You may have to ask at a Ford dealership to get that. I've heard some people can get them online but I don't know how that's done.
There's a few items that will need to be taken care of before they go bad, if they haven't already. So even if you found a truck that runs great now, you're going to have a minimum amount of work you need to do to it to keep it from having massive troubles later. It's a chain reaction kind of thing. Take care of some little things before they lead to big things.
Long beds have a 38 gal tank, short beds 29 gals. '05 starts coil springs in the front, shorter turn radius. King Ranch and Harley are special versions of the Lariat, top line trim level. XLT has cloth seats.
What else is there to know? I'm sure somebody will come along and help out.
You didn't mention if you've had a diesel before. This one takes 15 quarts of oil and most of us like the synthetic all year. It's special diesel oil, you don't use regular car oil. Most parts and service are more expensive than gas engines. With diesel more expensive now than gas, you kinda want to have the need for all that power, the better mileage doesn't really balance out the fuel cost anymore. Unless you're working it, of course, then it's way better.
Check the forums before you do anything. A lot of new guys jump straight into replacing things like air cleaners, put in the wrong oil filters, buying the wrong tuner, etc. because they don't read first.
If you can't find what you're looking for, ask specific questions, we'll help. There's a lot of good info in the stickied threads.