I've no idea of what faults can cause starting issues, but I can say that new glow plugs can make a world of difference.
My car started to be a bit 'chuggy' when started for a second or two, when it would just run a bit rough etc, it made the car feel a bit banger-ish lol.
So, about a year or so ago at the 200k mile mark I renewed all 4 plugs with Beru replacements (Beru are apparently the OEM ones) and the transformation was amazing.
Instant smooth starting in sub-zero temperatures , no more puffs of smoke on start-up.
The forum threads I read were full of doom and gloom about how easy the plugs are to snap-off and what a foul job glow plug replacement is etc etc but the job was a piece of cake.
The only special tool needed was a very thin-walled long reach 7mm (IIRC) socket to get the plugs out....