Have all the signs of leaky heater core and don't think the original has ever
been replaced after 154,000 mile and 17 years. Any advice or hints before attacking this problem and make of heater core recommended?

I just did that little task this past weekend on my wife's 2005 S80. I used the following document as guidance and it was fully applicable to the S80, so I assume it would be a big help on your S80.

http://www.v70xc.com/0/resources/how-to/pdf/heat_exchanger_removing.pdf

The only thing I did in addition was to flush the system before removing the old heater core.

Once you disconnect the hoses on the engine side of the firewall, you can take a piece of scrap hose slipped onto of the nipples and blow through it. It will push the majority of the coolant out of the core, making for less of a coolant spillage in the drivers footwell when you pull the core out.

Also, at least in the S80, it's impossible to R&R the core without bending up some of the cooling fins due to the angle at which you have to tilt the core going in and out.

I went the the cheapest of cheap heater cores from Advance Auto. Time will tell.....

I did mine last year with the Ebay Heater Core. Easy job, like Klaus said, drain the coolant, tilt the core aft and it's out, reverse the install. No issues yet!

Cheap is good. Look on Ebay for a replacement for $49 or less and free shipping.

Except for the contortions of being on the driver's side floor, not a difficult job. Lower the antifreeze level by about a quart or two, drain from the radiator into a clean container. After removing the kick panels in the car, place a leak proof bag under the heater core to catch a small amount of antifreeze that will come out when it is unplugged. Do not reuse the O rings from the original, use new ones.

Time for the job: 1-2 hours. There are a couple of Youtube tutorials for the 850, which is pretty close to your car.
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Keeping it running is better than buying new

I just did that on my s70. Took about an hour and we followed the 850 video and it's been working great. I went with the cheap heater core on partsgeek.com and so far so good. It's been about 6 months with the new core.