It can be anything really.
You need to read the fault codes to get some guidance.
Your engine historically has a few design faults resulting in stretched timing chain evry so often and leaking fuel injectors that lead limited oil circulation, destroying the turbo and the engine.
As there is already damage to the engine, it will be best to read the codes and not to try to push the car with the hope that it will clear out.
You can read the codes with Forscan:
https://www.google.com/search?q=forscan&ie=utf-8&oe=utf-8&client=firefox-bA turbo specialist can fix that but before you fix it, you must find the source of the problem, checking the chain via Forscan and the injectors and oil pick up.
It seems that you are in for a big bill but if you had the car some years and plan to keep it, the overall cost averages low over the years. If you just bought the car from an auction dealer, then there is a good chance that the previous owner had the problem and traded it in.