FYI....home warrantys will not pay to fully replace a heat pump. Most only pay a certain amount towards the replacement. Foreclosed homes are sold AS-IS, so normally no repairs will be done by the selling bank. I feel that you should have taken a look at the unit prior to making your offer.
You could try to negotiate a lower price at this point but most of the contracts that you sign with banks are iron clad and read that you understand that you are purchasing the home AS-IS and most of the time the only way out of the purchase is to have the home inspection clause in your contract read that you can opt out of the contract if the homes does not inspect to your satisfaction. If you have that option, you can use it as a tool to say...I will move forward with this sale if you agree to reduce the price by $5,000 as the heat pump is past it's serviceable age.