Another vote for fixing the civic. DH and I were recently considering replacing our 2005 Matrix with 205K miles on it, but decided we are going to keep it going for a while. It's running just fine. Well, the exhaust pipe to the muffler just broke the other day, so it is really loud, but we are going to see if our mechanic can weld a piece to fix it... fingers crossed! And it needs new tires. Anyway, the point is the cost of those things is not much compared to the cost of replacing it, since we expect to get many more miles out of it based on toyota's historical reliability records...
I raised the problem of being in a hurry to replace the thing if it goes unexpectedly, but fellow forum user pointed out that the amount I save using it for another year or two far outweighs the cost of renting a car for some time while figuring out what to buy, should it come to that.
$700 to fix the A/C is a lot less than getting a new car, and with a Honda you should easily get to 200K miles. Even 300K miles isn't out of the question. Also, depending on what is wrong with the A/C, maybe you can call around and see if it can be repaired for less? Look into the fix and see if DIY is a possibility?
How were you going to pay for the new car? If you drop $20K on a new car, you are also forgoing investment returns, so that's opportunity cost saved by continuing to use the civic.