If you wish learn Music Notation, then go for Harmony Assistant. It is easy and I learnt it within 3hrs.
But if you wish to compose Modern Music and Serious about sequencing, hi-fi Samples, then go for Cubase Sx, or Cakewalk Project or Reason from Propellerhead....
here is the link for Harmony Assistant:
http://www.myriad-online.com/en/products/harmony.htm
Download the Demo versions you buy any of the above software...
As far as Keyboards are concerned, there are n number of brands. Nowadays, a keyboard is hardly used for a Live performance and no longer a hot tool in composing and sequencing department...got my point...
Go for a USB Keyboard Controller...you've several...
http://www.musiciansfriend.com/key/navigat...07&src=3SOSWXXA
Bye...good luck...
Vijay Didla