Adding a payment button to the Webform Module

I have been administering drupal sites for a while now, but it looks like I need to get into module development for my next clients needs. Specifically I want to add a Pay option to the form, where a person will have to pay before the form gets submitted (there I need to integrate a payment gateway in the form ). Also after the submission is succesfull the person should be registered to the site.

Would appreciate any pointers on how I can achieve the above two goals. I was looking at adding a custom submit handler by using
hook_form_alter() . is that a good path to follow?