Using savings for D7 visa?

Good day all

Please can someone advise on how much savings is needed for D7 visa route if using only savings?
Also, my husband is South African, myself and my son are British and South African. If we are going to apply as a family do my husband and I both need to get NIF and do we need an account that’s in both of our names?
Can we apply as soon as we have savings in an account in Portugal?

Also, if I have a friend living in Portugal that can be my tax representative, do they have to create an online account on the Gov website and apply for mine through that?or how can they get it for me?

thank you in advance all!