Mechanically, a 300k mile drivetrain is going to be more problematic, than not. The 7.3l is a tank of an engine. Unfortunately, all of the accessories attached to it, not so much. It's also a discontinued engine. You can start counting the days until the parts are no longer available. The Gov't and manufacturers want the big smoggy bastards off the roads, so they're gonna do what they can to get them that way. In Texas, the gov't is giving away $3500/car, for cars 10 years or older, just to get them off the road. CARS. Imagine how badly they want the older trucks off the road... The 4x4 system is probably in one of two situations; neither is good. You'll find it beat up, because they used the absolute piss out of it. You'll be replacing U-joints, wheel bearings, hubs, potentially a transfer case, and ******, amongst other various seals, bushings, and hardware. Or, you'll find that it's hardly ever been used. This is HORRIBLE for 4x4 systems. Sediment settles in the transfer case, over long periods of time, and when you go to engage the system, it's gonna be like putting steel BBs in a garbage disposal. It'll bind the gears, chew them up, and clog a few oil ports, amongst other small issues, that ultimately, will equal 'BOOM', rather quickly, I might add. The automatic transmission attached to the 7.3l was not known for being an epic winnar.
Suspension wise, I'm sure the majority of it is shot. A cheap shock for that truck is going to be $50. You'll need at least 6, if not 8 of them, depending on the suspension. I'd hate to see the price of leaf springs for it, not to mention trailing arms, the pitmann arm has got to be ready to start crying, and I'm sure the steering box isn't in the best shape either. Ford is known for slacky steering, anyways, imagine the amount of slack that could build up in 300k miles, in a big *** truck with a couple hundred pounds of unsprung weight on each corner... especially seeing as how, the steering box is attached to the chassis (which moves with the body), instead of the axle (moves with the suspension, which is what it's hooked to).
Insurance, I'm sure, will be laughable, if you can even get it. I'm willing to bet I know where that price came from. It's what that person still owes on the thing. They've probably stepped back and taken a look at the costs of getting everything on it back up to snuff, and gave it the old "**** that shit, I'll get someone else to do it, and buy a new truck". I've been in the car business, for quite some time. I've seen PLENTY of trucks traded in, simply because the owner didn't have the time, money, or willingness to fix all the little shit that was wrong with it.
You're 17. Worry about being young, dumb, and full of ***. You can't afford this truck, plain and simple. I ca almost guarantee you, by the time you by it, and get it up to road worthy, you'll have over 10k invested. Then again, if you're stupid enough to (even thinking about) buy it, you're probably stupid enough to drive that big hunk of shit with slack steering, a shot suspension, shaft play in the turbo, and god knows what other untold problems.
Just sayin.