Sorry to hear that you are experiencing this, albeit it is not unique experience for a new owner. I recall that my first annual after buying my first plane also had a five-figure price tag, but mostly because add-on stuff I wanted to have done and few issues that the seller disclosed prior to the sale (price adjusted accordingly). On my second plane, I brought my mechanic to the seller's place who graciously provided hangar space, tools and support personnel for us to do a pre-buy. found couple of minor issues, agreed on price adjustment, shook hands and flew the plane home.
As for your options in pursuing the seller and/or his mechanic, that would largely depend on the terms and conditions of the contract. If the seller made any form of representations about the state of the aircraft's airworthiness and/or maintenance and these representations were, in fact, false, then you could have a case and creative lawyer could use several theories to seek damages or rescission of the contract. Conversely, if the contract terms were "as-is/where-is" with no express or implied warranties, you are probably out of luck.
I would be very concerned about the mechanic, though. His labor costs appear to be quite inflated and he may see you as an inexperienced pilot/owner representing a bag of easy money walking through the door. Seeking a second opinion from a mechanic with Mooney experience would seem to be a prudent course of action here.