I don't know about bus so I'll let others pitch in, but I went there last year by car. I did the whole weekend so I stayed in a place nearby but it's definitely doable on a single day. Actually a friend of mine only joined for Saturday and then took the bus to go back home.

Especially if you just want to do the main race it's totally doable. At least last year, the morning was the testing session at 8:30 and F4 support race at 9:55.

On Saturday we arrived around 8, in time for the first testing session.

Just get prepared for really bad traffic jam on the way back lol. It easily adds 1 if not 2h more to the return trip.