have done the same - put a td5 from a defender into my 200 tdi 90. much better engine, even in standard (non tuned) wiring is a bit of a challenge though - make sure you get the complete loom. will take you a while to understand what all the wires are for since lots of other functions (heating, airco etc) are part of the same looms - so its a challenge to work out which wires are needed and which are not.
the other thing that turend out to be a challenge was the fuel pump since i wanted to use the 200 tdi tank. the original td5 setup is wuite complicated so i experimented with inline pump from a bmw but that didn't work. I ended up copying the original set up with a lift and a high pressure pump as well as a swirl pot as well as the original td5 fuel filter. got the pumps from D44, that setup works well now.