Just google '850 heater core replacement' and you will find lots of useful and not so useful data. More of the latter.

Do the O rings fail? Yes, but usually it is the core. If I were you, I would pull the kick panel off the driver's side and take a look under the core. If there is antifreeze dribbling toward the carpet, the rings could be compromised. Regardless, replace the rings when you repl the core.

Prices are from $50 - $350, check Ebay for some prices. If O rings are not included with the core, then the dealer will have some for $5.

Do you need new firewall to core pipes? You will not know until you remove the core. Corrosion depends on the type of antifreeze used, the cheap stuff will mess things up.

Remember to use a plastic garbage bag to catch the coolant in the pipes and core. About 1 pint total. If you do not drain the block, then you will need to clamp the hoses on the engine side of the firewall.

Removing the driver's side carpet from the car took me about 15-20 minutes, I expect 30 minutes to put it back in.

Remember, the coolant always flows through the radiator regardless of the heat position. So after the core is back in, take the car on a test run to get the engine up to operating temps, park it and check for leaks after the engine is off and the pressure are at their highest.
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