What davec says. Emissions controls kill the ability to have a simple engine.
Common rail doosels are, as was also said, more finely controlled, so more efficient. But its this control gear, again as already said, that is more likely to fail in such a way that it can't be fixed by a home mechanic.
The 300TDi needs one wire to keep it running - mine fell of the other day
This is assuming the EGR is bypassed.
The Puma tranny engine WAS modified to put in the LR: The Dual Mass Flywheel was lost, cos it made the engine unstable at low RPM, the ECU was 90% re-programmed, and the oil delivery system was redesigned to allow it to run at greater angles.
That is probably why you can't get the bigger engine, cos it hasn't been developed to work.
My 300TDi has done 160,000 miles in a disco, i hope it does more than 15,000 in my 90!