I will add that it depends on what 200tdi you are planning to use.
A Defender 200tdi is simple, try to get everything you can from the donor including the radiator/ intercooler, glow plug harness, air cleaner and even the fuel filter if possible. Pretty much a case of pull the old, install the new and make a few slight changes to fuel lines and wires. Rover sold this option as a kit to upgrade the motor.
Disco 200tdi requires additional bits that are custom, you can purchase them premade. I think you might run into issues if you want to go LHD. Many threads on the web that cover the mods that need to be done and where to buy the custom parts if you do not want to make them