The M51 would probably be the easiest - more so than the M57 - and arguably a better engine. I would probably opt for the OM605, with a mechanical pump off the OM662, again for simplicity. However the OM612 is rather sexy... I like simple though. It keeps working. As you say, why change the TD5? It's pros and cons are well known and documented. Parts aren't stupid money. It's been proven to do big mileages when cared for. It's really not that complicated.
Parts are less well made than they were - some big problems with OE lifters, for example. But that's par for the course. Out of the box, the Disco TD5 is one of the best vehicles land rover ever produced. Very little needs to be done to make it tow well, drive off tarmac well, carry a family and all their kit for two weeks holiday, general run about.
Ive driven two 300 converted D2's. One was a Black Paw vehicle, fitted with a tuned 2.5 300Tdi. It was nice. At the time I had a 51 plate D2 auto, tuned by IRB Developments. I liked the converted vehicle. It was good. But it didn't set the world alight. My D2 was a blast. The difference in power train is just huge. The other conversion was by the company in Dorset. It isn't worth talking about.