It is not quite clear to me how you use the Playmaker (or intend to use the Essence) at the Moment.
The most common way - when we talk about a combination with a BS and speakers - is to use it a a 'source' connected to the A.Aux-in of the BS.
This way you can easily choose between e.g. playback of a CD and what is possible with the Playmaker/Essence.
Do you use a smartphone/iOS or just the Mac?
'How to set this up in PM':
Getting access to music stored on the NAS from the Playmaker/Essence requires a DLNA-setup.
On your NAS there already is a DLNA server (it maybe have to be activated first?).
You need a 'DLNA controller' (a software) to choose from where (= your NAS) and to where (= your Playmaker/Essence) the usic will play.
For smartphones there are several DLNA apps (one example is the PlugPlayer) - I mostly use the 8Player app on my iDevices.
Please google these and learn how to use them!
Most apps are even AirPlay compatible - which means that you can Airplay from them (to your Zeppelin or the PM).
There are similar apps for the Mac.
(Is your Mac currently hardwired to the PM?)
Only way to use AirPlay music (such as Spotify or TuneIN...) to multiple devices is to use an app like Airfoil on the Mac:
link
If you want to integrate your Zeppelin, this will be the only way!
But you wanted to avoid using the Mac ;-(
The strength of the Essence is that it has integrated support for TuneIn and Spotify Connect.
It can be managed through the Beomusic (iOS) app.
However, Spotify Connect requires a Premium account - but that is what you ought to choose, if you want descent sound from your BL4000 anyway ;-)
Asking about a 'fair price' for the Essence? I suppose you mean a preused one?
It will probably be hard to find a good deal on one - so buying new will be my suggestion.
Pleas notice, that you don't need the 'remote wheel', if the Essence is connected to the BS3200.
You use the Beomusic app to control it and you control the volume via the BS.
Complicated stuff - there are many ways to use an Essence!
How about exploring the possibilities of your existing Playmaker (DLNA and AirPlay) - and take your time to consider if you need the Essence?
Here is a place to start studying: