Yes! You should buy things – great idea!
If you like to download your music, you can get individual songs or whole albums, either in 192K MP3 format, open OGG format or in full quality FLAC. You can also purchase downloads from the iTunes music store and other online music stores.
Yes, there are karaoke versions for most songs – the two formats are MP3+G and MP4 (for iPod Videos). You get audio without lead vocals, plus on-screen lyrics.
Ringtones are a work in progress, I’ve just found a site that I think I like for doing this, and I’m working on getting some available there.
Finally, the merchandise page has a ton of stuff – shirts, mugs, underwear, not to mention coloring books and card games. If you can’t spend some money here, I can’t help you.