Learn Spanish (Latin America): Con un millón de dólares, ¿qué comprarías? - With a million dollars, what would you buy?

This is the conditional tense, used to talk about what you "would" do. To form it for regular verbs, you keep the entire infinitive (comprar) and attach a conditional ending. Because the subject is the informal "you" (tú), we add the ending -ías, making it comprarías.

Why is there a de between millón and dólares?

In Spanish, millón is treated as a noun, not just a number. When you are talking about a million "of" something, you must link it to the noun using de. So, "a million dollars" translates structurally to "a million of dollars" (un millón de dólares).

Why do we say un millón instead of just millón?

To say "one million", you have to include the word un. This can be tricky because for "one hundred" (cien) and "one thousand" (mil), you drop the un and just say the number word. Millón is the exception!

Could I add the pronoun tú to this sentence?

Yes, you can. The -ías ending already makes it obvious that you are talking to tú, so leaving the pronoun out is very common. If you want to add it for emphasis, it sounds most natural at the end of the question (¿qué comprarías tú?).