No obvious signs of worn out engine. I bought my wife's car with worn out engine and tried to avoid changing it . It was hard to start in early march. Fairly cold.Found 3 bad glow plugs.Changed them and no difference.
I assume the starter, battery and leads are good and when you turn the key it spins over fast enough to give good cranking compression? (Have you tried if it starts any quicker with good jump leads from a good battery on a running car, possibly with the revs up slightly on the donor vehicle?)
When it does start, is it lumpy as though trying to get on all four cylinders? (Possibly a faulty injector leaking allowing air in over night?) Not noticeable once running.
It doesn't start and die, then crank over until it runs? (I have had leaking fuel filters etc. that air gets in so the engine starts then dies and starts again lumpy but smooths out as the air is bled out by the system)
Regarding heater plugs, obviously you tested the heater plugs themselves as I would, usually just for continuity to start with and then physically to see them glowing red. The reason I say this is some people on this Forum have issues with the relays which changing cured their problem, though personally the heater plugs I find are a more common fault when testing.
I had an 03 non turbo Doblo non runner that another garage and a diagnostic company gave up on so I bought it for £300. There was a difference in battery voltage between the battery and the leads which I sorted , but the main reason was air in the system, as it was an early non turbo it didn't have a low pressure pump in the tank so no obvious leaks. I got it running with a direct feed from a can on the bonnet to the high pressure pump (only for a short test as it soon empties due to no return feed) the cause of the fault was an air leak at the plastic hose and it's connector at the fuel tank. Fiat wanted £200 for the joined flow and return pipes.
It was a poor design that relied on the high pressure pump "sucking" fuel all the way from the tank which allowed the problem to occur.