From a technology perspective, Mimo leverages quite a few advantages that non-mimo devices do not take advantage of -- ...http://en.wikipedia.org/wiki/Mimo
If your technology path must be the iPads, from a product perspective, they only support older models for a limited time. There will be new technologies and features that older models simply will not be able to take advantage of either due to battery or cpu constraints. Quite frankly, you have that issue with all devices, not just iPads.
Make sure you huddle with your peers and see what they are doing to ensure that you make the best decision based on the path that others have taken... 1000 devices is a big bet to make. Talk to others and see what they are doing...