I screwed up!! Advice please?

My take is you bought it over the phone and the buyer was not totally upfront about it, call it bad salesmanship. You do not entertain taking it back. How bad is it and an you make it all good and safe? That is now the question.
Like said, for $6000 you probably got what your $ would by you there. Not that often, that buys your a perfect totally done car.
You just wanted a safe driver, but cars in the rust belt seem to always have ISSUES I notice!! You or someone knowledgable just has to determine what needs to be done, and figure out who will actually do the required work.