Complex sentence: The Koestlers were relatively prosperous by local standards and lived in spacious, well-furnished, rented apartments in various predominantly Jewish districts of Budapest, large enough to accommodate a resident cook/housekeeper as well as a foreign governess during Arthur's early years.
Split into simpler sentences: The Koestlers were relatively prosperous and lived in spacious, well-furnished, rented apartments in various predominantly Jewish districts of Budapest. They hired a resident cook/housekeeper as well as a foreign governess during Arthur's early years.