My experience with 300tdi is that it's easy to give the engine a good flush out.
Assuming the water pump is in good condition, easy to pull it out. just get yourself new gasket.

Always handy to inspect a water pump anyhow, and if any rust on or around the impeller, better to replace it now .. rather than have a failure later. They're not stupidly expensive and good insurance.
Be warned tho, the 3 long bolts(around the round section at the lower side) are really long and can seize and snap.
I didn't know when the pump was last changed/removed/fitted on mine, I was extra cautious and had got those 3 long bolts before I did anything. Replaced when fitting the new pump.

Run some flush through the engine first, after some time then pull water pump.
I had trouble trying to remove the drain plug(pass side, near front) not easy access, so removed the dip stick to get better purchase on the bolt, but it felt like it was going to break. I didn't have my induction coil tool then. I just left it as is.

Remove the lower hose and you get good drainage from that.

IIRC the only part that won't flush properly is the heater core. I can't remember the flow now(years ago) ... I disconnected the hoses and back flushed it to try something. Water came clean but if you have any debris down in there, I think you'd have to remove the heater core.