Hi,
Sorry but I am not sure to understand correctly your request.
You can indeed display a product in a module, or have a link to a product page.
To select the quantity in a dropdown, you have to use set the option "Quantity layout on product page" in Configuration > Display > Product to "show select".
The option "After a product is added to the cart" in Configuration > Main > Cart can be set on "Always go to the checkout" to be redirected to the cart process.
When setting up a product in backend, you can attach files, and set them as not free to download, this way the customer can download the manuals only once the payment is done, and the file has no price, just the main product price is used.