Doubtful that this is the first time that basement has flooded....a primary reason why the previous owner sold this place. If mold is already present, that is proof that this has happened before, If you go down there for any more than a minute, put on a good Covid mask, so you don't inhale any of that mold. I assume you had that house inspected before you bought it..,.if so, they did a poor job.
You have little choice other than proceeding with the repairs, no matter what the cost. You will be lucky if the insurance covers this mess,
If you DO get all this fixed, I would strongly suggest that you sell this place, and move to an apartment....where the landlord will be responsible for any maintenance and upkeep.. After all...if you can't even mow the lawn, it's time to move on.