Hi @ManHoe1 ,
For your first question, if there is a monthly login 1,200 of External user (authenticated) on your side, you could purchase Tier 2 where USD 1,000 for 1,000 logins and a additional 200 logins with USD $200 Price/month.
The USD $100 Price/month for External user (authenticated) is not supported in Portals. Actually, the Power Apps Portals login capacity add-on Tier 2 (10 unit min) is actually the Power Apps portals login capacity add-on Tier 2 where USD 1,000 for 1,000 logins. If you want to purchase Power Apps Portals login capacity add-on Tier 2 (10 unit min) , you must purchase 10 unit at least ($100 * 10 unit => $1000).
Please check the following article for more details:

For your second question, these others 98 internal staffs with Azure Active Directory (AAD) could login in your Portal as authenticated users (if they have been added as Contacts in your Portal). But if they want to purchase SKU for external users (authenticated) logins, I afraid that there is no way to achieve this.
Actually, the PowerApps Portal License is acted as a Add-On Capacity in Power Apps License. If you want to purchase SKU for external users (authenticated) logins, you must own proper Power Apps license firstly. But if you just only want to access or login in a Portal, you do not need a Power Apps license.
In addition, for internal users, use rights to a “custom” portal are aligned with their “custom” Power Apps use rights. Please check the following article for more details:
https://docs.microsoft.com/en-us/power-platform/admin/powerapps-flow-licensing-faq#can-you-clarify-the-use-rights-to-portals-for-internal-users
Best regards,