Well, I don't know which mechanic has told you that Mitsubishi engines aren't reliable. In diesels you can only compare to Toyota, and there too, ask Saqib Bhai (Spectra) about the 2L-T, this engine's competition, and he'll tell you a bunch of interesting stories.
They are very much so, the thing is though, all mods should be done within reason. There is one issue that I have had with my high-miler, and that the engine is rather sensitive to oil changes. They should be done at the proper time, otherwise the engine suffers.
As for the turbo setup, except for a swap for a 4D56T 2477cc turbo engine, you don't have many choices. You could buy the injection pump, turbo, oil cooler, intercooler, and the manifolds separately to put on your engine, but the fitting would be a hell of a job. I'm not sure you could easily find mechanics willing to take on this job. Plus there are slight differences in the engine internals as well. It might cost even more than a new turbo engine as well...
Modifications required if you go for a "new" 4D56T engine should be minimal; with some mods for the oil cooler / intercooler (if fitted).
For an N/A engine, there isn't too much to be gained. You can increase fueling just a bit, the car will become slightly smoky, but will give a slight increase in power as well. Although in my opinion, it's a waste of fuel. I would suggest that you have the injection system properly calibrated, check to see that the exhaust and intake aren't blocked in any way. As far as I remember, even the N/A diesel Pajero is fairly respectable in terms of performance. Sure it won't beat new SUVs in a drag race, but it wasn't ever meant to either. It's a proper 4x4. 
Hope this helps, let me know what you decide.
Cheers!
Sami
