If I buy the Divinity: Original Sin 2 - Eternal Edition does this include the Divinity: Original Sin - Enhanced Edition Collector's Edition or the Divinity: Original Sin - Enhanced Edition?
It contains 1 copy of D:OS EE, with the Collector's Edition extras.
The other difference is that the D:OS 2 Eternal Edition does not include D:OS Classic. That is because it caused some confusion offering the original and updated releases of the game, there is less need for the original release now than when the Enhanced Edition was first released (one of the reasons it remained available with the purchase of the EE is due to the change in system requirements in Windows, with Classic being 32 bit and using DirectX 9c, while the EE is 64 bit and requires a GPU with hardware support for DirectX 11), and in general the EE being more up to date and a better experience (fully voiced dialogue, controller support, etc).
If you want the original release of D:OS, or a second copy for D:OS EE, you should get the D:OS 2 Divine Edition and D:OS EE Collector's Edition separately; that would include all the same extras (though cost slightly more, as the D:OS EE CE is a 2 pack).
If you don't care about the original release of D:OS or an extra D:OS EE key, then the D:OS 2 Eternal Edition would give you everything else in one package.