The Tdi was introduced in the late 80's and replaced in 1998 by the TD5.

The Tdi is a simple noisy , "agricultural" motor. But it works and is very economical

The TD5 is smoother , more powerful and loaded with electronics - the electronics are unavoidable on any modern diesel motor because of anti pollution regulations. Initially very problematical - later fixed and a good engine.

In 2007 the TD5 was replaced with the Ford based  2.4 l diesel, also with much electronics. A good , torquey motor that initially gave quite a few niggles.

Any of these motors that are checked and approved by a Land Rover expert will be good . You will battle to find a Tdi withless than 200 000km on the clock.

Most Td5's will have more than 150 000km 

And Pumas anywhere from 0 to 100 000km

Be aware that in 3 years time the Defender will be totally revised .This will probably enhance the value of old Defenders - and spares will not be a problem - particularly the Puma versions- . The true Defender represents an era and a way of life  that we will never see again - if you find a nice one get it.

Dave

ps Are you in Cape Town ? If so I can possibly advise  with sourcing and testing a  vehicle