If you want full control for the checkout, Magento is probably going to be the best option. You can also do it with woocommerce. The woo checkout has gotten better and you can tweek it to improve conversions, but you will get more flexibility (if you have a developer) with Magento. Saying which...
It is inevitable that you will see some rankings drop, no matter how well you check all the "migration best practice" boxes. I've never seen a migration done that resulted in no, or even positive changes right after launch. However, you can greatly reduce the time that your rankings fluctuate a...
If you aren't tech savvy, I would highly recommend shopify. It's designed with users like you in mind with a very simple back end that can run shipping, adding products and everything else you've mentioned. If you can run a wordpress site, you can run shopify. After setting up my first store I ...
Your question is very thin on details. You will have greater success of experts answering if you include more detail in our specific about what you're looking for.