300 is not any larger, it's the same engine, just refined a little more. 200tdi is nearly impossible to find parts for even in the UK but the 300 is very common... 300tdi is an easy swap into a series truck. Your bell housing bolts up with a few holes bored larger if I remember right. The 300tdi is a direct decendant of the 2.25 diesel, mounts are the same. If you are doing a 300tdi into a disco, bolts right in also but the R380 on the petrol has a different diameter mainshaft that will have to be changed. R380 from a diesel is a direct swap. I would go 300tdi, there is a reason it is so popular. 25mpg and the easiest swap... If my petrols ever go in my Discos they will get 300tdi's without hesistation... I did a Cummins 6bt conversion in a 109 and it was a ton of work making mounts, using different pieces and adapters-not easy... I have seen people use GM straight sixes (YUCK) and Isuzu Diesels- not bad... Just depends on how much time and money you want to put into it... There are plenty of write ups in the Rover Hybrid forums.
2003 Disco SE7 expedition prepped
1998 Disco LSE 7 50th anniversary Baluga Black
1996 Disco XD Trek number 4 and Camel Selections
1969 Series 2A 109