I'm creating a page where the user needs to input information about a widget into a form. The information needs to be detailed, as a results the form is very long , with 50+ fields (not all are required and the form can be expanded depending on features of the widget). The user will be charged a fee when submitting the form, and the entries in the form will be permanent, immutable. Any changes to the form will require that the form be resubmitted, the fee paid again and the previous instance will remain in existence but be superseded. All this to say that mistakes by the user will be costly for the user.

My intention currently is to "pre" submit the form, present the result as a mock-up and then ask the user to confirm each entry line by line, once validated by the user, it can be truly submit and the user will be charged the fee. But this seems very cumbersome and I fear it will cause users to abandon the form.

An alternate solution:
The form is broken down into 5 sections, require validation after each section by preventing the user from advancing to the next section until validation is complete. This will break it down into smaller parts making less daunting in terms of validation, but will stretch out the time/effort required to fill in the entire form, again leading to the same risk of abandonment.

Any ideas on how to balance the need for validation and ease of use? How can you get the user to double check their entries without overburdening them?