Hi!
- For this it really depends on your needs. I'd say check here and compare the features: https://www.agileit.com/news/understanding-azure-active-directory-licensing-free-basic-p1-p2/
- You do need to decommision the DC and point any apps utilizing the On-Premise AD to the Azure Active Directory. Also if you have any domain joined instances, you might want to enroll them in AAD or migrate them to Azure VM's (depending on your own requirements). For exchange do consider to migrate to o365 before decomissioning, if everything is migrated to o365, users might need to setup their outlook profile again (More info here: https://support.microsoft.com/en-us/office/create-an-outlook-profile-f544c1ba-3352-4b3b-be0b-8d42a540459d#:~:text=To%20create%20a%20new%20profile,profile%2C%20and%20then%20choose%20OK.) (also you might need to make some changes in the dns's txt (autodiscover) record and/or mx record when you're migrating)
- I think this video will provide some insights: https://www.youtube.com/watch?v=2dJu_AHVlzk
Hope this helps!
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