There are Sigma lenses for A-mount, however Sigma have stopped releasing new lenses for anything but Canon and Nikon.
Tamron however, have about the same lenses in A-mount as they do for Canikon.
Sony quality is not higher at all on A-mount, it's about the same as Canikon. The GM lenses on E mount is superior but on E mount, not A mount. Sony's prices are simply based on supply and demand.
A-mount or Minolta mount is not compatible with E mount, however, A-mount lenses can of course be adapted to E mount.
I love my A99II, the best Camera ever overall.