I'm not that hardcore into Atari gaming (if I want to play Atari, I'll just use my Gemini) but this is probably what you're looking for if you want to use paddle controllers on your Ouya:
http://home.comcast.net/~tjhafner/2600-daptor.htm
I've never bought anything through the google playstore, so I honestly don't know how that would work. Everything I've ever checked out from googleplay was free.
What games do you have that you want to transfer from google play to Ouya? Sideloaded games like that would show up in the builds section, not be integrated with the Ouya games.
Sure, if that suits your needs more, then you don't need an Ouya. but personally, I'd rather have a console than use a tablet, use the Ouya controller over the Moga (my best friend has one, and man is that d-pad stiff!) and I guess that I like the very concept of the Ouya being a small company's product that stands up in defiance to everything modern console games stands for with it's predominantly 2D based indie games and not the multi-million dollar blockbuster FPS titles on the other consoles.