Well, it sounds like you had concerns about the HVAC system before signing... they dropped their asking price, you liked it ... the HVAC system was looked at and received the stamp of "fine" form the service company. I'm sure the seller knew about the problems, but with a 27 year old system... you should have had someone come in and give it a serious look.
The seller signed, you signed... now it's your house and your having issues with a 27 year old HVAC.
I think you're going to need to budget for a replacement.
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