I wouldn't go back more than one generation. The issue is how long the device will be usable, and the older it is, the shorter it's operational life will be. The current iPad Air is kind of expensive, though. A basic model might be fine, as long as it's available with enough memory...
Ultimately, this is a feature that's more theoretical than anything. It also varies greatly by manufacturer and model. I already mentioned what is by far the best and most reliable method for sharing media, and that's AirPlay. It's built into basically every receiver these days.
I do that, but use the method John Dirk suggested, using the USB on a Blu-Ray player. In that case, I have an SSD connected directly to it. In the end, that's just a better solution than trying to network to a remote drive.