What is the ruling on someone who committed a sin (addiction to masturbation) and repented from it, but it left him with health effects: is he obligated to inform his wife of this when marrying, or should he not marry at all?

Do not tell anyone about the sins you have committed and conceal your faults, for the Prophet, peace and blessings be upon him, said: "Whoever commits any of these abominations should conceal himself with the concealment of Allah." Informing one's wife about sins committed is not permissible in Islamic law, and by doing so, one unveils Allah's concealment. So, rely on Allah and proceed with the marriage. If what you fear regarding the health effects of that habit has caused you impotence (inability to have intercourse), then you must disclose it, and she has the right to annul the marriage. However, if what you fear is not a defect that prevents enjoyment, then you are not required to inform your wife about it.