Selling iPhone - what do I need to do?

That is all you need to do.

Settings>General>Reset>Erase all Content & Settings...if you don't have a 3GS, make sure the phone is plugged into a wall outlet as this can take several hours(a 3GS supports hardware encryption, thus the erase merely deletes the encryption key). I'd also remove the sim and tell the buyer they have to obtain their own. Some people don't(no data is stored on the sim), just what I've done with every phone I've sold.

do i need to de-authorise the iphone?

No. You don't authorize or de-authorize the device, its your computer/computers that are authorized.

You don't need to remove the SIM since it doesn't store any data and the buyer will need it if he wants to use the iPhone as a Touch.

There has been no new phone announced in June at this time. If you get another iPhone, all you need to do is plug the new phone into iTunes and restore it from your backup.

As long as you backup the old phone, everything in the backup including SMS will transfer to the new phone.