I'm not a keyboard player but I believe that MainStage 2 will do what you need.
You can playback tracks from within Mainstage 2. You don't need another application like (iTunes, GarageBand, or Logic). You do it by creating instrument track channels that have an instrument on them called Playback. You can synch multi-track stems from Logic if you want.
The only thing i am unhappy with at the moment is that the bus/send architecture is still buggy and it only supports 8 busses even though there are 32 of the listed.
If you only want to playback a stereo backing track and use your keyboards and maybe a mic I think Mainstage 2 will work for you.
I am a guitarist/singer and I want to use Mainstage 2 as a combination front-of-House mixer, a Monitor mixer, and a multi-track playback engine. I have it doing all of that but if I copy/paste/duplicate a finished song so I can reuse all of my mappings of sends/auxes the new song/patch comes up with the sends all turned off and the only way to make them work is to delete them and re-add them which means you have to remap each control on the mixer view. trying to get a 100 songs set up is going to take forever doing it like that.
If they fix that and give us more than 8 simultaneous send/aux busses then I would be a very happy camper.
By the way, if you do want to use iTunes there are applescripts that come with Mainstage 2 that will connect the two together so that you can control iTunes from within Mainstage 2. Someone has also written a set for GarageBand that you can download off the web.
Also, FYI Logic Studio has a ton more instruments in it that are not in the Express version. Probably worth the price to a keyboard player to upgrade even if you don't use Mainstage.
Hope that helps you.
Dave