I think the issue with tdi's is that while they are nice and simple, most of them have done a lot of miles and even the youngest is 14 years old so you will pick up all the inevitable old car issues, rust dodgy seals etc. while the engine may run and run the rest of it may be falling apart around you. Mine is 17 on 146k and I need to replace the rear diff bearing and oil seal, gearbox and transfer box, cross member and a few other small things. not big or expensive jobs but still hassle.
Personally when the time comes I will buy a td5 as at least the chassis, and drivetrain will be fresh, as I have decided after a year of ownership I cannot be without a Defender.