Just a little more on the hoses inside the car, given the opinions already expressed in this thread: I doubt they need replacement.

One of my "ambitions" 15 years ago was to find a way to restore the thermostatic heater controls the 240 used before moving to the plastic valves. I collected them in the yards, sometimes taking the time to unclamp and remove the hoses, and other times just using a serrated knife to liberate them. In every case, the hoses looked pristine inside and out, and felt supple, in stark contrast to the look of the exterior heater hoses exposed to heat, oil, and engine movement.

Although kits are available to replace the valve stem seals, I never found a fix for the failing wax thermostat. However, I did learn something about how the environment affects hose life.

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Art Benstein near Baltimore

"Though it is true there are no stupid questions, it seems there is no shortage of false assertions to base them on." -Unknown

This is one of those questions only you can answer. It kind of boils down, in my opinion, to 1) how excited would you be about pulling the dash again in 6 months, a year, 2 years, and 2) how long do you plan to keep the car? Only you can answer those for you.

My 82's core failed in 2002. It likely has little to do with mileage on the car. Electrolysis is the culprit. If coolants aren't regularly changed/flushed then they can/do become capable of conducting current and the dis-similar metals in the system cause copper to be removed from the copper bits and deposited on the aluminum and cast iron bits. Remove enough copper - presto - you have a leak. If you've owned the car since new and the coolant has been changed/flushed regularly - more likely the core will last longer. How much longer? How long is a piece of rope?

If I were keeping the car, was in there anyway for the fan and wasn't the original owner of the car (or knew I hadn't done the job I should keeping coolant changed) I'd put a new core in it. And if you want to minimize the problem in the future - stop by a marine shop and fabricate a little sacrificial zinc anode to go in the cooling system. Future electrolysis will take its metal from the zinc before it starts back on the copper. Hoses? Again - keeping the car? I'd replace the hoses while I was about the business. That's me though. You get to decide for you. There's not just one right answer.
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82 242-6.2L; '17 Mazda3; '16 Crosstrek

IMHO - I would definitely replace the heater hoses. They are overdue. I wouldn’t replace the core. Maybe try reverse flushing the core while you are in there? The core on my old 90 was still going at 20+ years and 300k+ miles when I sold it.

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Will I buy another Volvo??? We'll see....

Maybe the plastic you were "hearing" about referred to the late-year plastic heater control valve.

Can't help you predicting failure. I love the old movie 2001 Space Odyssey but have to suspend disbelief listening to HAL predicting the failure of the AE-35 module.

I'd no more replace a heater core for preventive maintenance than I would get my prostate removed for that reason. Yes it will fail eventually, if the car lives long enough.

However, a blower motor replacement became a "while you're in there" prophylaxis.

You can read about me replacing a leaking core in a '91 here: Heater Core job
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Art Benstein near Baltimore

"Though it is true there are no stupid questions, it seems there is no shortage of false assertions to base them on." -Unknown