Learn Italian: Il vestito non è antiquato, bensì moderno, quindi compralo. - The dress is not outdated, but rather modern, therefore buy it.

Bensì means "but rather" or "on the contrary" and is used specifically to correct a preceding negative statement (non è antiquato). While you could use ma here, bensì creates a much stronger contrast, emphasizing that the truth completely replaces the negated idea.

Does bensì always have to connect two adjectives like it does here?

No. While here it connects the adjectives antiquato and moderno, bensì can also connect nouns, phrases, or even entire clauses. The primary rule is that it must follow a negative statement to offer a direct alternative.

Why is the pronoun lo attached to the end of compralo instead of coming before the verb?

Compra is an affirmative imperative (a direct command to the informal "you"). In Italian, object pronouns like lo (which refers back to il vestito) always attach to the end of affirmative commands to form a single word.

Could I use perciò instead of quindi in this sentence?

Yes, quindi, perciò, and dunque all mean "therefore" or "so" and are entirely interchangeable here. They are all natural choices for introducing a conclusion based on the previous statement.