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]

There are three things that can cause brown sludge in the coolant.

1. Mismatched Antifreezes: Chemical reactions cause additives to drop out of the solution and form sludge or slime.

2. Oil Contamination: from Blown head gasket, Cracked head. With a integrated Radiator / Oil cooler /Transmission cooler a internal breach with let oil mix with the coolant.

3. Corrosion: the additives in the antifreeze did not do their job. The most common cause is not periodically changing the antifreeze like one should. The Corrosion inhibitors degraded.

Looks like changing a heater core in a S60 P3 is relatively easy. You take the Floor console out and you can get to the Heat Exchanger which is at the head of the Floor console.