This question is probably not relevant to the topic.
But here are my thoughts:
If you want to know the truth, you should try to access (whether ownership or just access) to any devices that you intend to support with your applications. This, of course, is impossible for everyone, but there are ways in which you can maximize your value when creating a collection of devices, trying to hit the largest range of different types of devices.
You have no obligation to be locked in a contract (or even any service) for the phone. If you go to the carrier store, you can purchase the device at the full price and leave the store without activating any services on it (in general, they can be changed at the whim of any store you are in =) If you intend to call for the full prices, expect a price range of $ 380- $ 600. From what I can tell you about your situation, I think the phone is best for you. In addition to being able to buy from a carrier, Google also occasionally directly sells the Nexus device, which comes with Android (ish) android OS (no skins, etc.). The current device on this line is the <Nexus Galaxy, which is sold directly through the (former) Android Market. This is a good choice because it is more or less the latest generation of hardware and the latest OS (these Nexus devices, as a rule, receive their OS updates more timely than the devices associated with the carrier). In fact, the first Nexus device was widely sold to developers, this line of phones was intended for development.
Is the tablet PC the type of device I should get?
If you are going to create apps for tablets, then yes. If you intend to develop phone applications, you really should test on the phone.
Should I get a device with Android 2.3 or 4.0?
Ideal on different devices. If you need to choose one, then it depends on your budget 4.0, only now on the latest devices, so they will probably be more expensive than some of the devices that you can find with 2.3. (although $ 399 for the Galaxy Nexus can be hard to beat, even for some of the 2.3 devices for sale)
In addition to wifi, what else should I look for?
On phones that can be easily purchased in the United States, your options basically come down to a few things. Screen size (anywhere from tiny to fairly large size for the phone), HardwareKeyboard (in several different shapes and sizes), Camera (if you like it). In general, most of the other functions are fairly standardized (e.g. bluetooth, gps, accelerometer, etc.). Another thing to keep in mind is “Oomph,” even though they are called the phones people use today are small computers. They have a processor, a graphics processor and RAM, as well as a PC. Battery life is the last major divisor, there is a fairly wide range of expected battery life (tooltip, massive screen and multiple cores require a lot of battery) However, since you do not seem to want to use this device as your phone, some of these materials may not matter to you.
Are there any brands / models I should avoid?
Everything on the list of your first ebay link. In general (in the USA) the main manufacturers of phones are: HTC, Motorola, Sony, LG, Samsung. There are many other devices that are also good. But these guys, as a rule, constantly pump out the most used phones. Idealy you should strive to distribute devices manufactured by different manufacturers. Custom skins and add-ons that they like to use in their OS versions tend to like additional testing. One of them will help you help the largest site of your users.
Does it make sense to buy?
There are good deals you need to have on pretty, but slightly outdated phones. If you don’t know what you are watching, I would stay away, but it would be easy to get rid of it.