The later NEX models offer quite nice image quality, and - like any other MILC - they're good for adapted lenses. But in all honesty I think you're trying to fill a gap that doesn't really exist in your line-up, and if you want to use native mount lenses for a NEX then you've a whole additional system to buy for. I maintain two systems - Pentax and Sony A-mount - and I wouldn't recommend that approach to anyone. Far better to have just one system if you can get away with it (and most people can).
I'd stick with what you've got, and put the money towards any nice lenses you might fancy