Hi ColoradoJohn,
I use an old receiver to power my second zone. I have no idea how much power it provides. Unless you plan on playing those speakers really loud, it won't matter much. You can find an old receiver, amp or integrated amp on eBay or AudiogoN for well under $100 (think I paid $30 +shipping). It's hidden in the basement, and I never see it or touch it.
As for volume control, I have to means. First, I use the Harmony 890 RF-based remote, which allows me to control my preamp throughout the house, as well as outside. Second, I have a simple in-line volume control from RadioShack ($30) mounted on the patio. That allows me to control volume on the spot if I don't have the remote.
The car amp idea is a fine one, in general. You'll just need to be sure that your power supply can source enough current for the amp to run at the levels you need.
Good luck.