C3 100 2.5 TDI non-start

[Gerard]

I'm considering looking at a 1990/1991 Audi 100 2.5 TDi (C3 type-44 FWD manual) with the 1T engine. Seller reckons the ECU is faulty and is thecause of the non start situation. 

Before I head over there I'd like to get some tips on confirming it is just an ECU failure and not something more sinister. What can I look for and fettle with on site to pin the issue down? 

I'm not familiar with the TDI Bosch ECU let alone the turbo diesel engines. I've had some Hitachi MAC13A units for the 200 turbo C3 through here that have had failed Darlingtons that just needed replacement. What would make a TDI ECU fail? 

Gerard