The upside, in about 40 years this will be a really funny story you’ll have to tell. For now however, scary and frustrating. And expensive. Sorry that had to happen. I expect installing a used junkyard transmission will be the best solution. Going forward the common advice here when purchasing a used car is (1) check what Consumer Reports Used Car Guide has to say about the make/model/year, and (2) to always have your own shop do a pre-purchase inspection before writing any checks. The latter costs around $100 and imo is money well spent.