the problem you will face is the issue of Recurring payments.
eCart does not offer support for recurring payments, this would need to be hand coded and is not supported.
the PayPal eCommerce Toolkit can do recurring payments, but you will need to create an IPN page to set the user account if the transaction is accepted. more details on PayPal IPN can be found here:
Neither tool will get you _all the way_ through what you are trying to accomplish without some PHP coding experience.
If you would like help setting the recurring payments in eCart, or configuring the IPN page, you can sign up for a premiere ticket where we would work with you in a one on one scenario to set this up.