The 5th generation iPod touch is still the newest revision of the iPod touch. It's still so new that Apple must continue to support and will give the full version of iOS 7 to the device. I don't have an iPod touch so I haven't tested iOS 7 on the device. I'm sure others have and can answer the questions better. Keep in mind the betas of iOS 7 are beta, they have bugs, they aren't as quick, etc. Once the final version is released is when we can start testing stability, smoothness, speed, etc.
You can buy one now, but it might be safe to try and wait until late October. Apple might sneakily update the iPod touch without us knowing or hearing any rumours.