Hi Gideon,
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> Is portal necessary ?
You can install marketing application even without a portal available. which means a protal is optional when setup marketing application.
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/portals#run-your-interactive-marketing-features-on-an-external-site-portal-free
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> Do you need purchase a new portal license ?
Firstly if we want to use a protal, a separate Dynamics 365 portals license for each Dynamics 365 for Marketing instance is required.
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/portal-optional#choose-whether-to-use-a-portal-when-setting-up-a-new-marketing-instance
Secondly, depending on the Dynamics 365 for Marketing licensing plan you have chosen, your license may include a free portal (provided no other free portals are already in use on your tenant), or you may need to purchase a new portal license before you can install the Marketing app with the portals option enabled.
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/setup-troubleshooting#when-do-i-need-a-portal-license-and-how-can-i-get-one
Thirdly, you can have at most one free portal on your tenant, so even if your Dynamics 365 for Marketing license includes a free portal, you may still need to purchase an additional one if want to use a portal with Dynamics 365 for Marketing and you already have a free portal configured on your tenant (probably because it was included with another Dynamics 365 for Customer Engagement app that you are already using).
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> Should you deploy a test environment ?
If the test environment means "trial" version,
we should notice that a trials application can't be integrated with a Dynamics 365 Portal. Instead, they provide a special mechanism that lets you try out landing pages, forms, the event website, and other interactive features without requiring a Dynamics 365 Portal or external CMS system.
You could request a free trial application here:
https://trials.dynamics.com/Dynamics365/marketing
Trial version limitation:
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/trial-preview-limits
Hope it helps.
Regards,
Clofly