Ubercart - adding extra info from customer during order process

I am working on a donation feature to  allow a customer to make a variable donation and I have this working using the Ubercart Variable Price module. 

I have a couple of other requirements for with additional info to be provided by the customer making the donation, such as:

• In memory of (checkbox)
• If 'In memory of' is checked, display further inputs: name (textfield), relationship (textfield or select list), more info (textarea)
• Gift Aid (checkbox)

As well as this, it would be great if I could add a 'Checkout' button instead of an 'Add to cart' button so that they are not tempted to add other products during the checkout process.

Any guidance would be most helpful. Thank you!