Hi
I'm new here, and I just went through the agony of choosing between the 200tdi/300tdi and the TD5. I ended up going with the TD5, thanks to some good advice from other Defender owners and companies that specialise in tuning Defenders. I chose it because you can easily increase its power by mapping, blocking/removing EGR, changing the radiator, etc., without breaking the bank.
I'm not a tech and I don't know much about how the 200tdi would do with other changes, but if you get a good remap and fix the EGR on a TD5, you'll be good to go. One thing I'd say about pulling, especially 3.5 tonnes, is that you should make sure the engine and frame are both stable. If the back cross-member is broken or has already been fixed, you might want to move on to the next one.
Good luck with your search, and I'm sure someone else will be able to help you out soon.