I have a website that I'd like to convert to wordpress using this theme.

I'm looking to just have a large product image to the left, a "Buy Now" button and a price under the image, and a product description to the right.

I don't need an actually cart system, as the "Buy Now" button will link to an external order page.

Does anyone know of an eCommerce plugin that will allow me to do this simple set up, and without having to have an actual shopping cart system?

I would search for PayPal plugins at WordPress.org. There are several there you could try.