。
She thought a lot about what to spend the money on.
Literal
She [topic-は] that money [object-を] for-what-purpose use [question-か] [about-について] considerably thought.
何に使うか is an embedded wh-question — literally 'use for what?' — wrapped by について ('about, concerning'). The か at the end of the embedded clause marks it as a question being thought about, not a question being asked. Japanese loves embedded wh-questions for capturing internal deliberation: 'thought about [the question of] what to use it for'. かなり ('considerably, fairly') is a slightly stronger intensifier than ずいぶん, often paired with mental or evaluative verbs. The whole sentence walks through a moment of careful financial deliberation.