The guy at the petstore is WRONG. :/ And sadly, that is usually the case.
You do need a thermostat and you do need a hygrometer.
I would recommend not getting the snake from the petstore because as you said, they are force feeding them which you should never need to do with a healthy BP. I would search for a good breeder around you or see if there is a local reptile show you can go to. You will find that buying directly from the breeder you are going to get a healthy baby who costs half as much as those petstore snakes. Most people sell their normal BPs for around $25.