There are a number of different Microsoft 365 plans to choose from (as well as add-on products).
Let's concentrate on just those applicable for the 'up to 300 users' bracket for now.
You have four choices:
Microsoft 365 Apps - £7.90 user/month
Office on devices (Outlook, Word, Excel, PowerPoint, OneNote, Access, Publisher)
Office Online (Browser versions of Word, Excel, PowerPoint, OneNote)
OneDrive for Business (1TB of storage for each user)
No other services (e.g. no email, no SharePoint, no Teams, etc.)
Microsoft 365 Business Basic - £3.80 user/month
No Office on devices (but you can use your own copy of Office)
Office Online (Browser versions of Word, Excel, PowerPoint, OneNote)
OneDrive for Business (1TB of storage for each user)
Exchange Online (50GB mailbox for each user)
SharePoint (create an intranet, collaborate on documents, etc.)
Teams (Chat based workspace - like Slack)
Planner (manage tasks and teamwork - like Trello)
Yammer (collaborate & discuss across departments & locations)
Used to be named 'Office 365 Business Essentials'
Microsoft 365 Business Standard - £9.40 user/month
Office applications (Outlook, Word, Excel, PowerPoint, OneNote, Access, Publisher)
Everything else you get with Business Basic
Used to be named 'Office 365 Business Premium'
Microsoft 365 Business Premium - £15.10 user/month
Everything you get with Business Standard
Additional device management (e.g. remotely manage and wipe Windows 10 devices and mobile devices)
Additional security features & protection, like 'safe links' in emails to protect against phishing attacks
So which to choose?
If you use some other provider for email / calendar / contacts (and you're happy with that), and you have perhaps no more than two or three users, you could go for Microsoft 365 Apps.
If you have more users than that (or if you want to use 365 for email / calendar / contacts as well), you have a choice of Business Basic, Business Standard or Business Premium.
If you already have Office 2016 or newer, then maybe go for Business Standard for now - so you get all the services, but use your own copies of Office.
(I would then advise looking at switching to Business Standard in a couple of years - because it is one of the cheapest ways of getting the newest Office applications.)
If you've got an older version of Office, say 2013 or older - then I would recommend Business Standard.
Bear in mind, you can also mix and match - maybe use Basic for 'light' users, who can work on documents, spreadsheets, etc in their browser - and get Business Standard for the 'heavy' users.
Last but not least, if you have additional security requirements, or need extra protection against things like phishing and malware attacks, or want to be able to secure and manage your devices (laptops, desktops and mobile devices), then look at Business Premium.
For a full comparison of these plans, see http://bit.ly/2twXnNq
Note that the prices above are correct as of October 2020, are ex VAT, and are based on a 12 month subscription. You can pay monthly or yearly - the price is the same.
As an alternative, you can select a month-by-month subscription - but the costs increase slightly.
Note also that you can trial Microsoft 365 free for 30 days.
Contact me if you'd like to discuss this or any of my other blog posts