2016 s60 premier 4cyl overheating, sludge in coolant. Advice on how to change out heater core

[Walmekinder]

Purchased the car with 105k. 5k miles later the car would immediately overheat. My mechanic replaced the thermostat and then the electric water pump. Still had overheating issues and a gugeling sound from the heater core. Took the car to the local volvo shop to have it flushed. They decided it was a blown head gasket they performed a head gasket replacement and shaved the head. Should be 100 % ok after all that right....
Drove it all sumer without a single problem and boom, high temp, pull car over alerts. Now the volvo dealership says that they found sludge in the coolant system and I need to replace the heater core, expansion tank, and radiator for 4k.
Any one have a clue if this will resolve the issue. Why we would see sludge after having the head done and several coolant flushings about 8k down the road.
Also looking for advice on changing the heater core myself. Thinking of using aftermarket parts from fcp. I jave a bit of experience but am wondering how much of the dash I have to take apart.

[RickHaleParker]

Duplicate post the other is here. Do not respond to this one. Moderators delete this thread.