Assuming Walmart still sells the SIMs, it may be a good idea to buy a 521 and a prepaid SIM to test the network, and the phone itself. I'd let you borrow mine but I really don't see a need to do that given that for ~$100 you've bought a great music player if nothing else (microSD up to 64GB). Also on the prepaid experiment, be aware that there is no roaming, so areas where you have poor service could be improved with post-paid service (allowing you to leverage any roaming agreement that may be in place).
As a refresher, it's 100 minutes, unlimited text, and unlimited data (first 5GB is 4G) for $30 plus $10 for the SIM itself. I'm not sure you can buy this from anywhere but Walmart.
For in-service phone selection, I'd recommend the Lumia 521 for parental-types and Lumia 925 for everyone else. The screen is better (and bigger), the camera is WAY better, and you get better specs (which matters somewhat but not a lot). Sadly, no microSD, and they are all 16GB phones.
http://www.t-mobile.com/cell-phones/nok ... a-925.html
With the 0% payment plan, that's $21/mo over a 24 month contract (again, where the ETF becomes each phone balance). You can even do that with the Lumia 521, for $4.25/mo over 24 months.
So if you wanted no out of pocket (not counting the fact that your ETF is paid, but at this time it looks like you HAVE to trade in your old phones, but that's just profit since the ETF is paid), you could just add 3 521s and two 925s onto your bill. That would turn the $110/mo into $170.75. Which is still well over $100/mo savings. And I almost forgot! You get a 15% employee discount on the service (not the phones), so that would actually be $154.25/mo (before tax/fees) without having to buy phones out of pocket.