Looking to add an e-commerce section to a website

Looking to add an online store to an existing wordpress website.

The store needs to be re-designed to be aligned with the existing site - fonts, colours and images.

Needs to be adaptable for mobiles and tablets.

Paypal will be the only payment option (for now - will add other options in time).

We can add on products and categories ourselves.

Looking for the simplest solution which we can build upon further down the line.