That How to setup recurring billing for Secure Zone membership is the best guide we have at the moment.
Actually, what you want to achieve can not be done with the single form. If you read the article I linked to above you will find that when you setup a form for ongoing secure zone memebrship (you catually have to sell the SZ membership as this is the only way to get the recurring payments going) you will find that the form will render the radio button for every secure zone and these radio buttons will manipulate the amount. The inteval is set inside the secure zone and this is also something that can't be manipulated from the form.
This means that in your case you will need 2 or 3 different forms. You need to forms at minimum.
You can have these forms on separate pages or on one page and reveal and hide them according to the customer's selection.