Welcome to the Q&A Forum

I'm also going to recommend WordPress. It's big, battle-tested, and relatively easy to set up and use. They also get security updates out quickly, and your site will auto-patch itself if the security update is critical enough. Non-crucial updates are also very simple to install, (click a few things in a web interface). For the E-commerce part, WooCommerce is the big guy in the room. I'm also happy with WP e-Commerce, (disclosure: I contribute to its development sometimes), if Woo doesn't work for you. Shopify just launched WordPress integration as well, if that's more up your alley. As for SEO: Yoast SEO will do a ton. Also, if you really like code you can make WordPress output markup in pretty much whatever way you want without sacrificing the upgradability I started with, so if you're willing to go deep enough, it's, (to me, a WP fan), the perfect CMS.