1 - CS5 is no longer sold, but you may still buy Creative Suite 6
2 - There ARE differences between the 3+ year old CS6 and the up to date Cloud programs... how MUCH of a difference depends on the individual programs
3 - I am Windows only so can't comment on Win-vs-Mac
They're not going to buy anything new, apparently.
The main programs that would be used are Photoshop, Illustrator, Dreamweaver, After Effects and Premiere to my understanding.
As a Dreamweaver user, I know the code hinting alone is substantially different, not to mention the CSS tools.