You can currently redirect to another page after the user sends the form. So you could set up another page on your website with your PayPal buy buttons and redirect to this page when the form is successfully submitted. We will look into fully managing Paypal payments for a future version.
We have a PayPal guide for a simple customization that will submit a hidden “Buy Now” button which will redirect the user directly to PayPal after submitting the Quform form. Further customization is only possible with custom development, until we release full PayPal support.