get new water pump,
speedo, up to you. GPS is used by many but I doubt it would pass a roadworthy.
numerous oil leaks engine and gearbox and crankcase pressure. This one together with the difficulty in starting suggests a worn engine. If your battery is in good health and the engine spins up nice and fast then you are likely to be low on compression (worn internals of the engine). Do you wait for the glow plugs to do their thing or are you trying to start without glowing the engine?
Did the mechanics report provide individual cylinder pressures?.
300 Tdi engines can be had second hand but they still demand a good price and with a cheap unknown one you may not improve your situation significantly.
Here is a cheap one but it will need a bit of money to get the head sorted and you will still need the water pump as it is missing on this one.
This one needs more work and it does not come with a warranty. Wreckers will sometimes offer a short warranty if they are confident of the condition of what they are selling.
For the price of this one you would expect a warranty, within the 3mths as this one comes with you could drive it and test it to ensure it was not a total lemon.
With this one it sounds ok in as far as it has been tested but good luck claiming on any warranty (if the reference to Austria is a typo) although the shipping is not that expensive, just need to sort out what duty you would be up for.
You could call Hans on this one
We are getting a bit ahead of ourselves here with the replacement engine until you get someone else to check it out for you.

