Whilst I loved my 300tdi 110, car like noise levels and the smoothness offered by a td5 simply puts the defender in another class.
As far as I know the difference between a 10p and 15p engines are a different EGR setup, but as generally they get removed, this in no issue, also this coincided with the later flash programmable ECU which had the fuel map reworked to improve anti stall and jerky starts from standstill.
The electronic side of the TD5 system does add a level of complexity, however it is actually a simple system to understand.
As for a code reader, any reasonable independant parts supplier with workshop will most likely to check for faults on the ECU side FOC, given that they can then sell you a replacement part if you should need one, obviously best to call in when you would expect them to be quiet and have a word.
I would not use anything but fully synthetic oil on any modern turbo diesel engine, this is because manufacturing tolerances are so much closer than they used to be, this goes hand in hand with the life expectancy of a moder turbo diesel being in the reigeon of 200 to 300k miles and you simply would not expect such a return without adaquate servicing including appropriate oil quality