If you are looking for just an Android MP3 Player there are two options:
Galaxy Player 4.0
Galaxy Player 5.0
Both are pretty much the same except for screen sizes, one being 4" and the other 5". The 4" variant is for around ~$190, while the 5" is for around ~$220. Both have a 1GHz CPU as well as 512 MB RAM and run on Android 2.3 GB.
As for custom ROMs, xda has threads for Galaxy Player 4/5 and CM9 ROMs are available for them.
Keep in mind, these are like iPod Touches, (No Phone functionality).