Start with the easy stuff, and make each system work:
Put the bloody thermostat back in! On modern engines it's important for how water flows through the engine.
A diesel runs cold, so much so that many haven't needed the fan for years, and the thing is seized.
Is it actually overheating - that means blowing water out of the expansion tank. Check the reading from the ECUs sender, it's a different part from the gauge one.
Check the temperature into and out of the radiator, there should be a big difference.
Check the thermo switches for the fans(the AC one not working suggests a wiring fault. But you do need to check) and the associated wiring.
Is the system properly bled?
Have the coolant checked for combustion gasses, this is a big cause of engines pressurising very quickly. That means a head gasket, or worse, fault.
Some of this will require tools: a <£10 multi-meter is essential for the electrical faults; you might manage the ECU reading with a cheap OBD scantool; a cheap laser thermometer will do the temp checks and is also good for playing with the cat; combustion gas test is best farmed out.
Don't just swap parts unless you have proved they are faulty!