if we write 'Az orvos siet az ebédlőbe', the meaning is incomplete?
Yes, it is incomplete. We don't know if the doctor arrives there. We only know that that doctor is on his/her way to the dining room.
If it is "besiet", then the doctor is entering the dining room right now.
i would rather say " Az orvos az ebédlőbe siet " But both seems good to me.
as the noun in the object takes -be suffix, we should add -be suffix to verb?
You can add -be suffix to verb, but it's not necessary.
It is not that kind of relationship. Here they just happen to be the same.
can siet also mean to rush, or is there a better word?