a) Buona Domenica.
b1) Grazie, anche a te!
(b1) is our typical answer to (a).
Other possibilities:
b2) Grazie, anche a lei! (if you are in the /lei/ context)
b3) Grazie, anche a voi! (if you are talking to someone with his/her family nearby)
Note: in nowadays Italian /voi/ ALWAYS refers to plurality, don't use in the /lei/ context, referring to a single person.