Hi Andy;
You can still order an Elan if you wish. It's a great combination of features; and a popular body shape.
Essentially, the Elan would fall between the Essence and the Rogue/Europa. The Essence has only one preamp; the Elan had two preamps, one for each pickup, and the addition of the Q switch as standard. The Elan Plus adds the bass and treble boost/cut switches, which essentially makes it on par with the Rogue and Europa.
There are of course other differences in standard wood recipes, neck dimensions, pegheads, body shapes. But I think in general the electronics differences establish a basic hierarchy.
As for quality, all of the instruments are hand made to the highest standards with the best materials. An Excel may cost considerably less than a Series II, but the build quality is still the same high standard. The woods used may not be as expensive and the electronics package more basic; but those woods and electronics parts are still of the highest quality.