a number of cash payments would look quite normal, if you arranged a payment plan on paper but actually exchanged xthousand dollars for a vehicle all at once, it could be stimulating for IRS, or maybe DEA depending on exact situation
If it’s a “nothing fancy” car, maybe a 30k accord or less, and he has a good income, it’s not going to raise any flags with either state or fed.
Bottom line - pay your taxes and you’ll be fine.
I can’t emphasize enough how good a feeling it is to have the title to your own car.
--edit (a word)--