Would you suggest to get the oldest generation of iPod Touch for
testing purposes or only the 3rd and 4th generation?
The iOS 5 SDK has many new features that make app development easier. By the time your app is ready for release, most users will have moved on to iOS 5. It is my understanding that iOS 5 does not support 3rd-gen devices, so there's no point in obtaining those if you're going to use SDK 5.
