In Need of Car Buying tips/suggestions

So this little beauty is the new Chevy Sonic.  This is one of the three types of Chevy cars that I am looking into purchasing in the next few months.

I've been driving around for almost 7 years in a 2002 Mercury Sable that has been nothing but headaches for me for a pretty long time.  Let's put it this way, I have electric windows that haven't gone down in almost 2 years, I rolled 100,000 miles on the car and the engine started making a rattling sound, the power steering rack has gone out of it, I hit a deer with it and when I took it into the dealership in which I bought it they proceeded to keep my car for a MONTH, the AC belt went out of it, there is sooo much more to this story but I won't bore you with the details.

Basically, I'm in need of a new fuel efficient vehicle.  Did I mention that the fuel mileage on my beast SUCKS!  Also, there will be a baby joining our family in March so I'm looking for safety features as well.  And it would be great to drive around in a car that's not going to break the bank to maintain.

So here are my list of "demands" for my next vehicle:

1. Fuel efficient ~ I want Prius gas mileage (or close to it) without the Prius price

2. Safety Features ~ Can't be driving around the countryside in an unsafe car with a new baby

3. Not going to break the bank to maintain

4. WARRANTY! ~ I got screwed on this with the purchase of my current vehicle

5. Low monthly payment ~ $300 (310 at the very highest) or less to not break my budget, I don't mind paying longer to stay within this range.

So here are my questions to you: 1. Do you have any other tips/suggestions for me to use? 2. What do you drive? Would you say it's fuel efficient and safe? 3.  Is there anything else that I forgot?