HI,
The creative Zen is not supported on the Mac, there is an old un-released, un-supported driver from creative, which enables you to access your Zen from iTunes, but it does not work reliably.
I don't know about the SanDisk or the Rhapsody service.
My brother in law has done some research and it does seem that the iRiver should work on a Mac, however you will not be able to use iTunes to automatically manage your player.
The easiest is by far an iPod, if price is a concern, look at the refurbished, a colleague of mine has bought a couple and is very satisfied with his purchases.