Are you looking at Office 365 E3/E5 or Microsoft 365 E3/E5 as these are different beasts.
Office 365 E5 is the top level SKU, including extra products (Power BI, Telephony, and various security products for example) that many users may well not need.
Office 365 E3 is the SKU that most tend to choose - it includes a locally installable copy of Office and the various services such as Exchange/SharePoint/OneDrive/Teams etc. that most users need.
There is also an Office 365 E1 SKU which includes Office Online rather than a locally installed copy.
Finally, there is an Office 365 F3 SKU for around £3.00 per user per month that features cut down versions of many of the services (2GB mailbox limit for example) and is limited to use on devices 10.9" or smaller.
In general, this is a bundle of Office 365, Windows 11, and EM+S…all of which come in E3 and E5 variants.
There are also Microsoft 365 F1 (£1.70 pupm) and F3 SKUs (£6.00 pupm). While F3 features cut down versions of most of the products across the Microsoft 365 suite, the F1 SKU doesn’t include Windows and has a very small subset of M365 features. You can see more info here - https://www.microsoft.com/en-gb/microsoft-365/enterprise/frontline#office-SKUChooser-0dbn8nt
Bear in mind that M365 F3 is different to O365 F3!
There are also F5 SKUs to add-on security functionality to the M365 F1/F3 SKUs.
E5 users tend to be the high end users but it’s really important to work out if you actually need E5 at all. Many of the features can be added on to an E3 license at a lower price.
Another thing to watch with E5 is that many of the services activate at a tenant level so if you purchase E5 for a subset of users, ALL users will be “benefitting” from some of the E5 specific features. Microsoft have detailed technical steps to block (most of) these.
E3 will be the bulk of the knowledge workers.
E1 could work for users who don’t make heavy use of Office.
The F SKUs tend to work for non-office staff - drivers, factory staff, front of house staff, field engineers etc.
However, as James said, this will depend on your specific org and users but that’s a general guide.
Push to Talk is a feature within Teams and is included with all paid licenses that include Teams so there’s no extra cost for that.
Hope that helps but any further questions, let me know.