Please consult a qualified and experienced audiologist in your area, he is the best person to guide you regarding the suitable hearing
aids for your father. The hearing aids you purchase should cover the hearing loss of your father.
Buy a brand that is having good services, there should be some trial and return policy. Try the hearing aid that best suits your requirement, then only purchase the hearing aid. There are few hearing aid verification tests your audiologist can perform for you, that can determine the suitability of hearing aids.