Both engine swaps will require engineering because, unless I'm mistaken, in both cases the engine mounts have to be changed.
I'm not sure I've understood this, but aircon shouldn't influence your motor choice one way or the other; compressors can be fitted to both 300Tdi and V8. Irrespective of what motor you use, you should be after the Australian-only 300Tdi or TD5-era Defender under dash A/C, not County A/C. County A/C would block off your dashboard vents, and I think you wouldn't want that.
...then again, I wouldn't fit A/C at all and I think the 90 in that Wheeler Dealers episode was far nicer before they got to it :(

