T5 2.5 turbo diesel transporter with oil in coolant bottle

[Kimbo surf]

Hi new to VWW
i am in Newcastle Australia and having trouble getting a repairer to diagnose if it is oil cooler, water pump, head gasket or crack in the motor. Car still runs fine and although the coolant bottle looks black(possibly oil in coolant), when you take the lid off and look in you can see easily to the bottom of the coolant tank. Anyone have some answers.
Cheers Kimbo

[Umai Naa!!]

Oil cooler on the front driver's side of the motor would be a good start.

Is the engine oil over full on the dipstick? Like WAY over full?

There's a cover on the gearbox end of the motor, towards the back, when removed, may reveal the condition of the water pump. The gear will seem loose due to bearing failure in the water pump.