Is it possible to have a custom downloaded text/ringtone?

Sure it is, but it has to be a compatible ringtone for the iPhone and it has to be added to iTunes and then synced to the phone. You cannot download to the phone, except ones that are purchased directly from iTunes.

Try this I've been doing this and it's sweet.

http://www.simonblog.com/2010/09/06/how-to-create-ringtone-for-iphone-on-itunes- 10/

Also you can always purchase ringtones in the iTunes Store app directly to your iPhone.

Hope this helps.