SharePoint server Subscription Edition License

Dear All,

I am using currently SharePoint 2013. I am planning to Migrate to SharePoint server Subscription edition. Kindly clarify the licensing details.

1. What license do i have to purchase? TroyStarr : Mentioned that Software assurance is required to use SSSE. Only Software assurance is enough? 

2. Do i have to purchase Software Assurance + M365 Cal license?

3.  Kindly specify M365 plan which can used for license?

Regards,

Gunesh v