Is it available for customers without iPhones?

You'll need an Apple device to set up Family Sharing (for the 5 users part), but if you don't need that you can set it up in a browser.

Fortunately one of my 5 has an Apple device.

I haven't gone through the process yet, but it additionally looks like I will need to borrow a Mac or Windows machine to start up iCloud Email. Once it is enabled I should be able to use IMAP or their web client.

Okay, so I have to count the device cost in. And I don't see an official Android client in Google Play. Seems like showstopper for me.

From what I've read, on Android you just go to Settings / Passwords & accounts / Add account / Personal (IMAP) and you can use ICloud email through IMAP.

That said, if you don't have anyone in your family with an Apple product, this iCloud+ thing may not be right for you.

Rackspace has a 10 user minimum.