iPad1. No other option.
How old do you think it's reasonable to support tech for nowadays? Bear in mind that the iPad1 was released just three and a half years ago, and was then updated a year later. iOS 6 has only been around for just over a year. So are you suggesting that the maximum life span for a $500-800 piece of tech should be 2 years? Or 1 year? I'd protest at even 3 and a half years, personally.
Mail app worked out with Imap, but still!
But, the ipad 1 is already 4 years old. That is pretty old when you consider how quickly the market moves.