Using Sonos with an iPad

No, your iPad will continue to work as a controller, it just can't be a place where the music is stored.

I think you'll find the rest of your questions answered here: main thread on this, which offers, and discusses many options, ranging from free to not?

David,

If you only want to stream music, then you can continue using the Sonos controller on your iPad and your current speaker setup to do that, as Airgetlam said.

If you want to play your own music stored on your iPad, then your plan will work fine. I know this, because it's exactly what I just did. I bought one new speaker, the Sonos One, and added it to my three older Play One speakers. (Also with a bridge setup.) I can start AirPlay on the new speaker, and then all the speakers will play with it as a group.

I hadn’t considered this as a solution to the end of the ‘on this iphone’ function.
So if I just bought one new Sonos play speaker (to go with my older playbar & play one’s which don’t have AirPlay functionality) I could use AirPlay to access the music on my iPhone?

Would I need to select that new speaker first each time to play to & then group the others in, or can I have them grouped from the start and AirPlay will cover them all even though it’s only on one speaker?

When I first set up the new speaker, the music only played to that speaker. As part of my initial setup, I then created a group with all my speakers, listing the new one as the default speaker. Once I did that, the other speakers automatically play once I go into AirPlay mode. (Assuming I have them selected to play as a group in the Sonos app.)

Only the new speaker with AirPlay actually shows up in the Apple Airplay control panel. But once the music is playing, you can control the volume for the group or individual speakers the usual way in the Sonos app. (I'm assuming that if I bought additional AirPlay enabled speakers, they would also show up individually in the Apple AirPlay control panel.)

You assume correctly. Airplay only 'sees' Airplay enabled speakers. It knows nothing about the Sonos groupings

Yes - pretty much the new speaker then becomes your "hub" to get airplay music to any older sonos speakers you want.