I don't see much all that much difference between Android devices
that you'd need to test against. After you've made a decision of which Android version
to target, it comes mostly down to hardware differences. Different screen sizes can be handled by the emulator, and with respect to features such as camera, phone, sensors - either they're present and you can use them, or they're not. Either way, not much to test.
Picking a minimum version of Android to require depends on which features you absolutely need
for your application. 2.1 is in widespread use
by now and gets you probably 80% of the overall market. But it's not like there are a whole lot of versions to choose from.
Also see http://www.businessinsider.com/tweetdeck-steve-jobs-2010-10