Cetane in diesel, not octane. Decent fuel is a good idea, but you could run 'city' diesel with an additive like Millers Diesel Power Ecomax. I would advocate running some cleaner through the system first, lots of Redex is my first 'go to' product but others might have alternative suggestions.
With the starting issue, try turning the ignition until glow plugs go out, then turn off ignition and repeat twice more without starting. Then start on the fourth go. I've used that 'trick' which was shown to me by a 159 owner on my own 147 and it works just fine; especially at this time of year. A whitish smoke is not at all unusual at this time of year too.
Have you checked the battery recently?