To be honest, if I were looking for a DD, I'd pass on anything with over 300k on it. You'll hear the diesel craze of "oh 300k is nothing for a Cummins" which yeah, could be true. BUT, there's no vehicle that makes it to 300k without having some kind of problems. Steering/suspension like tie rods, ball joints, track bar, steering box, pitman arm, sagging springs, shocks, etc. Drivetrain such as axles and seals, ring and pinions if they've never been done, transmission, transfer case, if it's an automatic then torque converter, if it's a manual then clutch. Random things start to fail at those miles. Brake booster, master cylinder, injectors, pumps, hell even turbo's have wear items in them. That's not to mention the interior from 2nd gen's. Seats, dash, trim pieces.
Sure, the truck could be a diamond, or it could be a diamond in the rough. Having a reputable shop check it out is smart, but with that kind of mileage that truck is best left to a farm where it'll spend the rest of it's lift living in the fields.
I just don't understand the diesel craze I guess. People willing to spend money on trucks with 300-400k on them for daily drivers, 90% of the trucks never see trailers hooked up to them. Why spend the extra money? More expensive to fix, more expensive to maintain, I can have 3 oil changes done with my 360 for the price of one diesel oil change. They do NOT get better mileage than gas engines without quite a bit of money put into them. People pay more money just to "roll coal" and put 8 inch lifts, 33 inch tires stretched onto 22 by 14" wide wheels. If you're building a performance truck, you're not buying something with almost 400k on it.
Again. I'd pass. Not my opinion, just facts in this case.
If you want my opinion; if you have no use for a diesel, don't buy a diesel. Reading through this thread tells me a couple things: You don't have a ton of money to spend since you're considering a 15 year old diesel truck with almost 400k on it. If you're that tight of a budget, then once parts do start to fail, you'll have a hard time keeping up with replacing those parts. 53 block or not, that's a ton of miles for any vehicle. That motor more than likely doesn't have that much life left, and once a diesel goes, it's not a quick cheap fix.
That's my opinion about this. Sorry if it hurts any feelers, and of course I could be dead wrong about the truck, and you as well. Let us know how it works out, and if you do wind up with the truck, we'll help you out along the way, whether it's customization, or repairs.