Hiragana is just fine. If you want to write in kanji, 叶う is correct.

1-夢はかなう is fine. it is more popular to say it in hiragana かなう than 適う or 叶う. you can also say 夢は実現する. it sounds more polite and less casual.

ドリームズ・カム・トゥルー。 ドリカム。