Learn Spanish (Latin America): ¿Ustedes comprarían un yate bastante caro? - Would you all buy a quite expensive yacht?

For regular verbs like comprar, you leave the verb in its full infinitive form and add the conditional endings directly to it. Here, we add -ían to comprar to match ustedes, creating comprarían (you all would buy). All regular -ar, -er, and -ir verbs share these exact same conditional endings.

Since comprarían means "you all would buy," how would I say "they would buy"?

It would also be comprarían! In Latin American Spanish, ustedes (you all) always shares the exact same verb conjugations as ellos and ellas (they). The pronoun ustedes is included in this sentence to make it perfectly clear who we are talking about.

Why doesn't the word bastante change its ending in this sentence?

Because bastante isn't modifying a noun here; it is acting as an adverb meaning "quite" or "fairly" to describe the adjective caro. When used as an adverb, bastante is invariable and never changes form. It would only change to the plural bastantes if it were directly describing a plural noun, such as bastantes yates (quite a few yachts).

Why aren't the descriptive words placed before the noun, like they are in English?