Used Model 3s are all way overpriced right now. While KCapital is right in principle that $43k is too much, this is one of the lower prices I have seen for a used long range M3, especially with FSD. The prices for used M3 will drop significantly when the Model Y comes out, just like the used Model S prices dropped a lot when the Model 3 came out. Tesla owners like to have the newest tech regardless of the model. If you need a car now, I would buy a new or inventory Model 3 that is in your price range. If you can wait a year or so, prices for M3 will be drastically different, which is something to keep in mind for your own resale value if you buy now.
Some early builds did have flaws, but not all had problems. If you get this car, I would do a careful inspection and have the owner provide the Tesla service records. One last thought, with 30k miles the owner has burned though most of your warranty. You may need this even more with an early build, which is why I recommend buying new yourself, but if you want THIS car, see if the owner can purchase a Tesla extended warranty for you at his expense.