I suggest you purchase the M80s, then the 500 and the centre last. In the meantime, use the speakers you have to complete your HT.

The Onkyo will have no trouble running your M80s. For reference, I have an almost identical room volume and I listen to music with a half watt per side and movies with a few watts. That leaves plenty of headroom for dynamics.

Good luck. I am sure you will enjoy your Axioms.