Not new to Subaru, but have Q's

[Legobrick2000]

Hey all, just picked up a 2000 Legacy GT sedan and am having a smell of coolant INSIDE the car. It's my indication that this means HEATER CORE LEAK. How difficult is it to replace??
It's in behind the center of the dash, correct??
Also, it has code p0325,knock sensor. Would that cause a sluggish acceleration??

TYVM
Happy motoring!!!