Dynamic Form Fields

I need a dynamic form handler/creator - not sure if that's the best name for it so let me describe what I need a bit further. I want to create a feedback form that will start with some required fields e.g. name, address, e-mail etc, and 10 or so check boxes. When the user checks one of the boxes additional form fields will automatically populate the page, so if they only select the first check box then only the first set of questions will appear on the page... If they they select for example the 3rd check box then the page will automatically refresh, whilst retaining any information already entered, and the third set of questions will be added. I need to be able to define the check boxes and associated fields myself either through HTML or PHP (I am a real PHP novice but should be able to manage it). I also need full control of layout. I need to be able to install the script myself with minimal effort. When the form is complete and the user clicks 'submit' I need the form to be sent via sendmail to me as well as a copy sent to the user's e-mail address they entered in the form - It would be nice if I could also append a short message to the start of the e-mail the user receives. Finally I need the user to be redirected to a thank you page. It doesn't matter to me if this is created from scratch or a series of pre-existing scripts are combined to create it. It also doesn't matter to me whether this form is handled through a MySQL database or JavaScript. The budget available for this is unfortunately miniscule at only $20-30 but I plan on having some much bigger projects in the near future and will be looking to tap any relationships formed from this.