Where to start? Ok first off don't buy any adapters for anything yet. Read this and get a little depressed, first (but less depressed than if you wasted money on an adapter that isn't compatible with your device).
Now let's make you feel a little better. Hardware wise, yes Nexus 7(which has Tegra 3 chips) should be fine with Miracast.
Although we don't really know if the bandwith of the wireless card will allow for streaming content from the interwebs to your TV, what the quality will be for a 2.4 Ghz wireless card or if Google will natively support it for the N7 (even though 4.2 is supposed to)
Time will tell. For now, just sit back and watch the posts to see the early adopters complain or rejoyce. As it is we really haven't even seen Miracast in action in the real world, just very controlled demos intended to get us all excited (please correct me if you have a real world video).