We want to use the Marketo Forms Engine to create Donation forms. Has any one else done this?
You can't use only a Marketo form for this, since it doesn't have any payment processing ability.
You can integrate a payment widget (my preference is Chargify) into a Marketo form using the Forms JS API and the widget's own API. See these posts: Using a form to accept credit card payment/or an e-signature and Can you accept payment through a Marketo form?