Which engine do you have, two options on the Punto Evo the 1.3 or the 1.6. as you're in greece both where quite common I seem to think but the 1.6 was rarer in some markets.
My 1.6 was quite smoky when it started up, it had a large exhaust system and I think it used to get a lot of condensation which would then turn to steam as the car was started so you got a lot of "white smoke" out of the tail pipe especially in the winter.
The glow plugs on these should give a warning if they are not working by flashing at you. Temperatures are Low in Greece today I see so plugs may kick in, but for the most part on these little fiat engines they will not preheat and the engine should start perfectly fine without in warmer temps Do you get the same smoke in the summer, or is this something that is only cropping up in cold weather?
Your mechanic isn't sounding to be very helpful if all he is saying is "some valve must be faulty" or saying compression is low.
As an example you could have 4 cylinders with 1)199psi, 2)199psi, 3) 197psi 4)199psi, in this case 3 is low but it would make zero difference.