December isn't really "impending". I don't think many vendors would be happy, or smart, to agree to such a long closing period (16 weeks+).
What happens if there are changes or the market falls further? You could walk away from the deal and then where would they be? Presumably they are selling in order to buy somewhere else themselves, where does such a long closing period leave them?
If you're convinced stamp duty will be abolished then take that amount off your offer and close today.