Need custom form for checkout on Marketpress

Hi, I am helping a client develop a site with products that will require an extensive medical form to filled out. I have the form in a plugin called "Forms" and is located here

I have market press installed. What the procedure needs to be is
1.) Client selects product
2.) On checkout a medical form is presented and filled out
3.) An authorize only payment is taken and sent in

The medical form is processed by a doctor who calls the patient.

How can I best do this?