23 year old furnace? Not a problem if the house is at least 23 years old. What were you expecting? If the house is 5 years old and someone has installed a furnace 18 years older, you have some grounds for complaint.
San Diego’s climate is very mild and furnaces aren’t used heavily. There isn’t that much to go wrong and they aren’t made of plastic. It isn’t that common to replace them. It probably isn’t really near end of life.
Barely works? If it requires maintenance or repair, you should expect the seller to fix it. If it is undersized or the ducting is substandard and it won’t heat the house, you have an issue to work out with seller.
A lot depends on how motivated the seller is and how good the deal is. Your agent seems to feel “not that motivated”….
