Hi,
I'm an amateur myself when it comes to all this, but my understanding is this...
Firstly, I don't think you can rule out the Head Gasket given that you say it sounds like it may be running on 4 or 5 cylinders. Whilst the filler cap is a ready indicator, it isn't a definite way of confirming one way or another - for that, you would need to check the compression on each cylinder and also I suggest check the coolant for exhaust contamination (there's some kit that can do this apparently). Hopefully though your assumption here is OK
As for the smoke, I've been led to believe that most "plumes of smoke" incidents are generally turbo-related. I suppose that knowing the colour of the smoke would be a help as this can give a clearer indication - i.e. white, blue, black.
As a novice, I'd hesitate to try to diagnose such a problem myself without more info, but (and I'm sure others can correct me if I'm wrong) I'm guessing that this could be the oil seals in the turbo that have failed. If that's the case, then it's definitely repairable
Good Luck!
Tony