Is going to the doctor and taking medicine an obligation, or is not seeking treatment when able to considered suicide? And what about preferring to leave money for one's family instead of spending it on treatment, believing that lifespans are in God's hands?

Seeking medical treatment is permissible and recommended, and it becomes obligatory if refraining from medication leads to death. Refraining from treatment during illness is not considered suicide. Furthermore, abstaining from medication to save money or under the pretext of it contradicting reliance on God is incorrect; because preserving life takes precedence over money, and seeking medical treatment does not contradict reliance on God, as the Prophet, peace and blessings be upon him, used to seek treatment. Medicine and means are part of God's decree. A Muslim should utilize the means while knowing that the outcomes are from God's decree.