Our focus is fast and reliable performance, the ability to call our CRMs APIs, no transaction fees on the platform, and have the ability for a user to login to see past orders and other info from our CRM. Our checkout process will need to be web forms that use a particular vendors API. I'm not sure the best cart and storefront functionality. Right now we use a shopify front end and pass products via UTM parameters to the checkout page. We don't use Shopify as the transaction fees are pretty high when you take into account upsells, and the other plugins we'd need to operate. We're thinking about migrating to wordpress and woocommerce, so we can integrate wordpress users to offer our customers the ability to login to the store, see their passed orders, and other information.
We have about 10k visitators a day, 500 sales a day, with another 500 interacting with our site as adding to our email list. We expect to hit 20k+ visitors a day fairly quickly (within 6 months).