。
She advised me on which one to buy.
Literal
She [topic-は] which [object-を] should-buy [embedded-question-か] me [to-に] advice [object-を] doing-gave.
Three layers stack here. First, an embedded question どっちを買ったらよいか ('which one one should buy') sits as the topic of the advice. Second, ~たらよい is the soft 'should/it would be good if,' less prescriptive than ~べきだ. Third, ~てくれる frames the advice as a kindness done to the speaker. どっち is the casual contraction of どちら ('which of the two').