There's no price in your post.
The iPhone 3GS is the 3rd iteration of the iPhone and was released in the summer of 2009..it is essentially the lowest iPhone you want to go since going lower than that will mean newer versions of iOS will cut you off..
I've got the iPhone 3G and iOS 4.x is a bear on it..it works, but not as fast as iOS 3.x used to..
Also look into iPhone 4 depending on where you are and what carriers around you are providing..
Regards