When should users register and pay for content

Complete newb here and apologies for such a simple question…
My aim is create a site that users can pay for the privilege of filling out specialized legal type forms more easily than accessing the forms on the government websites. I’m intending on having a reasonable number of different forms for different needs. I guess it’s basically selling digital content. Designing the forms, and the content of the forms, is not the issue, that part is under control. But one important factor in this is that any information entered by a user must be confidential and secure from other users at all times.

What I’m not sure on is the best way to set up the site in terms of registration and payment.

  1. Do people normally make users pay at the beginning by first registering with the site, paying the money at the same time, then directing them to the appropriate form? This seems the more secure way to do it. Or,
  2. Give users open access to any and all forms letting users complete the form first then only pay at the download stage? This would allow user to see if they really want to purchase the completed form.
    What do you experienced people think?
    Thanks for your tips