2. Get a Zone Bridge (about £40 now I think - just gone down in price). This is a box (small) that sits by the main Sonos unit
No it doesn't, for one thing there's no such thing as "the main Sonos unit". The Zonebridge doesn't have to sit next to any other Sonos device, it just needs to be within wireless range of them (otherwise there isn't much point having a Zonebridge).
ONE Sonos unit needs to be hard wired to your home network, that can any Zoneplayer or it can be a Zonebridge. You can also use a Zonebridge wirelessly to extend the Sonos mesh network, without it being wired to your home network (that's how I'm using mine).
and wirelessly beams music from the base ZP90 station to the Pla3 or Play5 units throughout the house
Again incorrect, if the Zonebridge is wired to the network any other Zoneplayer will use it as the final hop to the music source (whether that's a NAS, PC or the internet), said Zoneplayers will use the most efficient path they can find (ideally, this isn't always the case) to the source and will generally ignore any other Zoneplayer if required.
If you actually have a Zonebridge sitting right next to a Zoneplayer and either one of them is wired to your network then you really don't need the bridge at all.
You also don't need to plug the NAS directly into a Zoneplayer, it can plug into the home network and work fine, although connecting it directly to a Zoneplayer can help in certain situations if a router is struggling to cope with the data load between the NAS and the Sonos devices.