Stock head unit is fine with upgraded speakers. I had them done shortly after buying the car. I don't remember their specs, but they did sound better than the stock speakers.
Be aware that some Versa options packages had a 6 speaker configuration with separate tweeters in the A pillars. You'll need to disconnect them if you want to use standard speakers. Just look for a bunch of grille holes drilled in the trim above the tiny window.
I just bought a new stereo from Crutchfield and they sent a PDF guide on replacing all the stereo components (stereo, tweeters, door speakers, subwoofer, etc.) It has really detailed instructions with photos and diagrams. I can send it to you if you like.