In that case, definitely see the Sonos site
You can see the various options of equipment that may suit your needs. It should be inside your budget for the most part.
Adding (separate) wireless speakers instead of a whole unit (like the Sonos system does) would be a different animal- closer to a real AV receiver. Long term though, you might want to throw up/install simple pairs of speakers in your deck areas if you don't end up with a portable unit like the Sonos (etc.)