Doesn't even make sense, imo.
1. If you even find a "low mile" IDI, it's still plenty over 100k, plus going to be way more than $5k.
I hate to say it, but a 300K mile IDI is still a perfectly drivable truck most times. And I've gotten my turbo trucks(admittedly not in the best condition visually) for $2K or less.
3. You're forgetting about the whole drivetrain/suspension on the IDI that's going to need replaced. Not to mention any rust issues. (Again, rust free are out there, but that's going to raise the price immensely.)
What's going to need replaced again? Based on my experience, you might need a few bushings in the front end if they haven't already been replaced, shocks(routine), etc.
The front axles(at least the Dana 50s) seem to last just fine as long as there is oil in them; transfer cases don't disintegrate badly, Sterling 10.25 rears are pretty bulletproof overall, and even if you did have to replace the ring and pinion, that's maby $200 in parts, and a couple hours?
Rust... At least where I am(some salt equivalent used on the roads), you might find a little rust, but things don't 'rust out' like they do back east. My '93 has quite a bit of body rust, but it was a Canadian truck. And the frame/drivetrain/suspension is still sound!
4. Most of the parts you mentioned the 6.0 or 6.7 needing because it's 200k miles. False, unlike the IDI, some stock fuel system components will last well over that mileage. Especially the common rail parts in the 6.7, which is the most reliable system out there.
Until you get a little water in there... New fuel system time!
An IDI on the other hand... it dies, you drain the water out, bleed the system and it fires right back up again. No real harm done.
Worst case, if you got water in there and it sat for a long time... replace the IP($800) and injectors($300)... Done deal, nothing internal to the motor.
I do not believe that a common rail system is the 'most reliable' system out there; mechanical injection is far better in that the pressures are a lot lower and you have no computer to fail.
No, it will never get the fuel economy of a common rail system(LB7 duramax in a 2500... 20.8mpg average recently. Wow.), but it's a lot simpler overall.
I mean... no matter what we think about the DB2 IP, it's a relatively simple design, with few moving parts. Not a ton that can go critically wrong, which is also why IDIs get this reputation - instead of just dying, they keep running worse and worse as things wear out... but they still /run/.