It should be good to about 20 degrees Fahrenheit, but weigh around five or six pounds. Well, it depends. I keep wishing for a Toyota Land Cruiser, but so far, no go.
Bags are easier.
On the low end, price-wise, take a look at Cascade Designs' Cascadia. It's a rectangular bag rated to about 25 degrees that weighs a reasonable 4 lb. I don't see how you can roll over when using the hood. I guess you just sleep on your back, mummy-style. It's really a nice bag, and at 4 lb.