Google Mail/Calender/Workspace or what it's called. Works perfectly in a web browser and you can connect Thunderbird or Evolution. Most people who use Linux professionally are on Google. And most other people, too. Microsoft is a niche player unless you are locked in.

You can add self-hosted and open-source stuff anytime, but you'll need good email and calender that plays nice with the outside world, so Google. Maybe something like mailbox.org. Maybe you ISP's email + Thunderbird + nextcloud + K9 on Android will do, but you do not want to self-host or experiment with email if your business relies on it.

Nextcloud works fine for internal file exchange (but so does google drive), internal calendar, many apps on top of it. It even has a web-based mail client, but personally I would not use this is as my primary business email unless you are a hosting pro.