is the 'na' here necessary?
yes. Either that, or you can say 'tutakuwa wenye afya', but there has to be some connector there, since 'afya' is a noun and not an adjective
Yep, as machieng said, afya is the noun "health".
Tutakuwa afya = We are health. (?) That doesn't make much sense.
Tutakuwa na afya = We are with health. = We have health. = We are healthy.