As yours is a diesel, it will definitely not be headgasket failure as the engines are pretty solid. I think I've only heard of 1 HGF in the two years or so I've been on here.
Things to check for the smoke will be:
1. Intercooler O-rings at the bottom of the intercooler hose as mentioned above. Also check that your intercooler hose isn't split.
2. Blocked EGR Value, this can cause it to stick as a lot of black gunk gets stuck in there. I think a blocked EGR can damage your turbo. Can replace for a EGR bypass.
3. MAF sensor, might be worth unplugging the MAF to see if it makes a difference.
4. PCV Valve/ Crankcase Breather filter - may be worth changing this as it is a serviceable item which is often missed. A blocked filter may cause excess smoke I'm not sure but one clear symptom is if oil is being pushed out the dipstick slot.
Those would be my main pointers. All can be searched on this forum, plenty of info on them here.
Also, in regards to your engine temp not getting past 1/4 mark, this will be due to a stuck thermostat which is a common issue. Thermostat gets stuck in open position. This can be fixed with the inline thermostat mod which is also documented on this forum in detail
Lastly, make sure you have checked your plenums on your cdti to make sure they are clear, as the ECU is located in the plenum on the passenger side and can get flooded if the plenums are blocked. Quite an expensive fix if that happens.
Rick