It is by far the best option to start your priest as a premium character from the outset, the reason being that all your characteristics will then be rising the whole time you are raising your Faith.
A follower can become a priest regardless of premium time(this is not true anymore), the reasoning behind is for players to try out priesthood or experience a little of the enchanting.
A basic account is limited to 30 [[faith]] and 20 [[favor]].