I understand both the 200tdi and 300tdi are 2.5 engines, but what are the other differences between the two? Do 200 have viscous fans and 300 have electric fans on the rads? Any info appreciated.

Both have viscous fans. There's not much difference, 300tdi was an evolution of the 200. It's quieter and more refined but the 200 is much more robust and generally the favoured engine. 200tdi's also seem to get a bit more mpg but this varies

There's some good information over at Land Rover Toolbox's channel on youTube.
Land Rover Toolbox Videos - YouTube

No EGR or CAT (If i recall correctly) on a 200tdi, whereas there is on the 300.

I look at my 200 and think 'I'm glad I have a 200 as it is more simple, more reliable and easier to fix than the newer engines', but as I plan to use it mostly on the road I also find myself thinking sometimes I might have been better of investing a bit more and getting a td5 (300 sits between the two but it nearer the 200 than the td5) so I could tune it a little ...

Horses for courses

Tom