The 2.0D I believe has the standard 2C (or 3C engine in later models) which are very common and straightforward motors. Both of these can be effectively overhauled from a good diesel mechanic of which there are plenty in Karachi. Cost of 45-50k is reasonable and result should be good enough for another 150-200k. I'm not in favour of replacing the engine on a 2.0D as apart from khwaari you cannot put in a 2C-turbo because of limited space and would be raising eyebrows when you go to sell the vehicle. So find a good mechanic and inshallah all will be well.
BTW I cannot believe the dealer qouted 175k for an engine rebuild!!!