Complex sentence: The key figures behind the purchase of the helmet-crown were Grand Vizier Ibrahim Pasha and his chief advisors, İskender Çelebi, the chief treasurer, and Alvise Gritti, a powerful merchant who dealt mostly in jewels in Istanbul.
Split into simpler sentences: The key figures behind the purchase of the helmet-crown were Grand Vizier Ibrahim Pasha and his chief advisors. These advisers were İskender Çelebi, the chief treasurer, and Alvise Gritti, a powerful merchant who dealt mostly in jewels in Istanbul.