Well, actually, you *can* buy the phone on eBay and not get the data plan.
If you own the phone outright, they cannot require you to take a data plan...they can only do that if you get a discounted/subsidized phone and agree to a 24-month contract. They make back the money from the discount/subsidy on services, which are more expensive on a contract plan.
If you bring any phone to T-Mobile that you own, you can sign up for a plan and get any features you like. Heck, you can call them and do it on the phone or on line, and they'll just send you the SIM card.
Now, let me tell you why doing this is really stupid economically.
First, you will pay top dollar for a new Android phone. The MyTouch, for example, costs $400 new from T-Mobile (I know, I bought two of them in March). You're not going to find them much cheaper on Ebay. Even a *used* MyTouch or G1 is going to run you $200-300.
Second, the point of the Android operating system and the phones is to use the web. Many of the functions on the phone won't work without a data connection. In fact, for an Android phone, you have to have a Google account on the web and you need to use it when you first activate the phone. The phone will not activate without it. If you did manage to get it activated (no, you cannot use wi-fi to do it), all the phone will be able to do is make calls and maybe send texts (if you have texting on your plan). Which makes it a very expensive brick that makes calls.
Finally, let's say you get the phone, get the data plan, activate the device, then drop the data plan. You mentioned $1.99 per megabyte for data. Do you have any idea what your bill will look like after even some moderate data downloads? The average mp3 file is 3-4 MB, so that's six or eight bucks right there. You need to be attached to the web to get apps, and since most of them run about 1 MB average, after downloading a few, there goes your bill. Even browsing web pages or using the GPS sucks up a lot of data.
This is why they offer a data plan. It may seem like a lot until you stop it and have to pay as you go for data. It will kill you.
I feel for your financial situation, but what you're proposing to do makes zero economic sense.