The AD2 engine is pretty useless without the "bought" kits.  You can download the AD2 engine as a demo...but it's just that....a limited demo.  Once you purchase a kit, you have a useful piece of software.  I'm not exactly sure how it works as far as the actual kits that come with the AD2 upgrade (Studio Pop, Studio Prog, and Studio Rock) as I purchased the upgrade from AD1 to AD2.  I am assuming that if you just purchase a single kit...that is the only full kit that you get....but you get the engine to play the kit.  And if you purchase an upgrade from AD1 to AD2, you get the studio kits as well.

What Ted said... when you purchase a specific kit, you get that plus the "engine" to play it.   When you purchase AD2 itself you get the "engine" plus the delivered kits (which are nice) and the ability to add other kits as you go (like the Fairfax).  Since AD2 is now include with X3P, the Fairfax kit would be your purchase if you chose that route.
 

Makes sense, guys, thanks.  I am always in need of better kicks and snares and was thrilled to upgrade from EZ Drummer 1 to 2; but if you've seen my other posts, you will know that was a disaster.
 
I sometimes listen to 20-30 kicks before picking one for a song.  I don't like boomy, but I also don't like splatty or too hard. :-)  Picky, right?

konradh
Guitarprima, Thank you.  My question was about which expansion packs were best and I appreciate your answering.
 
Others, Although I am aware of the upgrade option, I have reason not to do that at this moment.  Thanks.

 All AD expansion packs now include AD2.