Parts kits won't be available I am afraid - just not cost effective (it would cost more for the parts than the fully built board I reckon).
And of course, soldering BGA packages is not for the faint hearted and really quite expensive (the little film mask thingies you use to line up the packages are not cheap). I've tried and you need lots of special kit. Just soldering on resisters etc at the sizes used is decidedly unpleasant!
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Contrary to popular belief, humorous signatures are allowed.
I've been saying "Mucho" to my Spanish friend a lot more lately. It means a lot to him.