You can set up Messages for Bonjour messaging and disable iCloud. That should do what you want. It's in the account preferences for Messages.
There's no such thing as a app that can receive messages without running. They can run in the foreground or background, or have a helper running instead, but something must be running for push notifications to work. The memory cost is negligible.
Hi wolfman! Thanks a lot for your suggestion. I tried what said and it works great. Didn't realize Messages allows screens-sharing and video-chats as well. Couple of things though:
1) I added Messages to my Login items and did the same on the other two MBPs in my household. But, how do we prevent the Messages window opening up everytime we log in? It's kinda annoying to have to close it each time.
2) Don't see an iCloud option to disable. The Messages account shows (inactive). I suppose that's the one?