Where do I start?

A play 1 is a very versatile unit; it can range from a cheap and cheerful single player set up all the way to a multi unit sound system as good as any traditional HiFi set up, with the Sonos Sub to aid in that direction. So the 1 is a very good building block.
Buying one unit to start is a good way to building the system you plan to; the one thing that Sonos must have to work well is a stable Home Wifi, so be sure that is present.
The other thing to remember is that by itself the 1 unit will not play music from old - or new - CDs; nor with it accept bluetooth supplied music from a smart phone. The former will need more supporting kit. But if your listening sources are services like Spotify or Apple Music or others; or if you plan to listen to internet radio stations, it will work very well to play music from those sources. Even just a single play 1 unit will serve quite well. You won't get the stereo effect that a pair will give of course, but it will go surprisingly loud, while still sounding good.

Thanks for your reply. I'm wanting to use it mostly for movies from a smart tv and music from our phones. From what I get from what you say, downloaded music isn't going to work but using music streaming apps like Pandora should work, have I got that right?
What kit would I need for cds to work? Is there anything that I could add on for music on my phone to work or does that just not exist?

If you are wanting TV sounds you need the playbar as it can be adjusted to sycn the tv lips to voices.
You could plug your cd player into a sonos connect, or rip you cds to pc/laptop/NAS, or upload to google play music (free) and stream from there.
Only the sonos app and spotify app can be used, Pandora app is coming next.
Sonos can play music stored on your phone, but if the phone leaves the house/off music stops. Best to use phone as a controller ie it tells sonos where to get the music stream from.

So in your opinion Paul, what is the very minimum that I need to get started?

A PLAY:1 or pair for music.
If you buy direct from sonos website you have 100days to try it, then return if it does not do what you want. If you have a large budget then go for playbar for tv as well ;O)
Remember you are tied to those apps. So no bbc iplayer radio, no sounds from you laptop speakers(unless line-in like play5 and a wire), no youtube music, no Bluetooth, no take down the beach, you need a good wifi signal in your house.
But they are great streaming radio or from music service, press button on top and streaming music resumes ;O)
You need to make sure hubby buys into sonos as you will want more, its expensive, not the best for tv surround sound, but the easiest, flexible and a lot less wires.

He mentioned it to me a while ago (he's the tech head in this house! Hence my need for guidance- if I ask him, he will cop on that I'm planning on it) so I know he's into it.
Would you say a playbar and a play 1 will work well in the meantime?

ok just checking, men are quite particular about techy stuff.
You really need 2 x play1 to act as surrounds (rear speaker effects) with the playbar.
That leads on speaker locations, stands, and power sockets, don't forget all sonos stuff needs a power lead.
Maybe just start with 2 x play1s, I have 1 in kichen & 1 upstairs in bedroom. Then you can try grouping for same music upstairs & down. Then later on he can decide if he wants to go down playbar route and move them. There might be a new paybar next year, but that is a long long long long shot, sonos never tell.

Is the playbar 100% necessary for tv?

Yes.
It is specially designed for tv with single optical cable from tv.
Some people use a cheaper sonos connect with play1s either side of tv, but it is not recommended due to controlling lip syn, and you will only ever get stereo sound.
But the playbar has some techy issues getting true DD5.1 surround, namely whether you tv passes DD5.1 down the optical lead from other devices plugged into it, many do not.
It may give hubby a techy challenge ;O)

Paul you've been a diamond, thanks so much for all of your advice!