Complex sentence: The Teylers Stichting had him design a prize medal in 1778 that is still used, and after his death they purchased the stamp press for it from the Holtzhey estate for 100 guilders.
Split into simpler sentences: The Teylers Stichting (English: Teylers Foundation) had him design a prize medal in 1778 that is still used. After Holtzhey's death the Foundation purchased the stamp press for the Teylers prize medal from the Holtzhey estate for 100 guilders.