The Sonos One/Play:1 speakers and Playbar are all compatible with the S2 App.. see this link for product compatibility.
https://support.sonos.com/s/article/4786

All generations of the sub will work. I would personally choose the slightly newer gen2 if both the ‘source’ and ‘price’ were okay and if I later wanted a dual sub HT setup, then I would get the gen3 sub next time around, that’s just my own personal preference.

Hi @SarahNP 

All of your components are compatible with S2. Is there a reason you have not moved off of S1?

To answer your question you’ll have to somehow determine if you are buying a 1st or 2nd Gen sub as there are no markings on the box. The retailer may have them marked as a clearance item. I doubt you’ll find any 1st Gen subs in retail stores anyway. 

Edit: I type to slow. @Ken_Griffiths answer is correct

Hi @Ken_Griffiths and @AjTrek1 

Thank you for your replies!   Ken_Griffiths I am not quite sure what you mean by dual sub HT setup, I am afraid I don’t know a lot about Sonos and the different setups. But do I understand you correctly, when I assume that you would recommend getting the Gen 3, as that is the ‘smart choice’ in terms of possibilities in the future?

And I had no idea I could just switch to the S2 app, I will do that now :)

Dual sub is now supported in the latest S2 Sonos App. It means you can attach two subs (one must be a gen3 sub) to a Sonos Home Theatre setup - so you can now bond your Playbar, with two Sonos Ones (as rear surrounds) and now add two Sonos subs to that setup too - nice if you want plenty of extra bass shaking the room, but lookout for the "for sale" signs from the next door neighbours.

@Ken_Griffiths  Hahaha great, thanks!