I plan to get a used no-contract device in order to start understanding the platform.
What would HN'ers suggest as a good representative device that a typical 25-45 yr old Android user might have?
A typical 25-45 year old Android user is a pretty wide open space dude. I have a HTC OneS but I also had to buy it out of contract.
1. Get a device because you will need one for debugging. Some things can't be debugged in the emulator. The easy thing is to buy a Nexus 4 in the Play store. That's a great price for a really nice device. If you have the money, get a 10" tablet, too. Asus tablets are very nice.
2. Understand that no device is adequately representative of the devices you want to target. So don't assume anything about screen size or density, and don't assume that if it looks good on your device, you are done. Make lots of emulator configurations. Make very large screen configurations. Rotate the screens. Change the font size preference. You would be surprised how easy it is to break apps by changing the font size.
What you will learn form this is that you can't make a separate layout for every device. Don't try. Don't limit the user. Let them change screen orientation. Let them change font size. Let them run your app on a 22" landscape mode display connected to the HDMI port.