AB, the M80s are slightly above average in sensitivity and are quite easy to drive. The amount of power needed depends primarily on your listening distance and how loud you want it at that distance. With typical listening distances around 10' or so and at comfortably, but not dangerously, loud levels, any power rating in the 100-150 watt area(very little difference between those two numbers)is plenty and shouldn't be a cause for concern.
To keep under that $600 number there's an outstanding buy at this time on the Onkyo 809 from ShopOnkyo here
. There's a Father's Day sale on all factory refurbished units with 17% off for the next six days. The $699 price shown on the 809 is reduced to about $570(shipping included)with the 17% off and a $10 credit for registering for "Club Onkyo". There's also a 5% credit(about $29)given which may be used on a future purchase.
The 809 is an excellent quality unit which fits your needs, including one front and seven back HDMI inputs.