Starting Problems 2.5 TDI Engine

Hello!

I'm new here and I'm having the following problem: My A4 2.5 TDI, manufactured in 1998, with a 6-speed transmission, always needs about 5-7 seconds of cranking before it starts, and then it emits a white cloud of smoke.
The preheating behavior is strange: sometimes it preheats at 2°C, but mostly it doesn't, and sometimes it doesn't preheat even at +2°C.
He doesn't need any coolant, and there is no oil film in the expansion tank.
It produces some smoke when accelerating hard, although not excessively.
I noticed that he experiences a slight drop in performance between 2300 and 3000 RPM. If he's been parked on a highway rest stop for about half an hour, the performance on the acceleration lane is very poor, but it normalizes quickly.
Fuel consumption isn't that cheap: around 6.5 liters per 100 kilometers with very gentle driving. A similar figure of 7 liters was also achievable with an older BMW 524td (E28)!
I'd like to get your professional opinion first, before I take it to an expensive and potentially unsuccessful specialist workshop...

Thank you in advance.