Hi,
I will describe some of the scenaruos which I would perform for a standard Magento2 store - Your probably have much more functionality :)
1. Shopping process
From adding cart to the cart, through applying discount, to finishing order proces.
2. Search - depending on what solution have you choosen this will define what need to be tested. For me it's the second most important feature in web commerce.
3. Customer - all features related to the Customer: registration, reset password, view invoices, change settings, add review and many more
4. Email communication - standard email and newsletters
5. Admin panel - this also needs to work flawlessly. I will focus on any custom solutions, but also I will test most important Magento parts (orders, product and category management, configuration)
After I will finish testing you will receive precise report which will list all found issues and list of all passed tests.
I will know everything after you will show me the project.
Thanks,
Chris