After reading several reviews, I decided to buy the Ubiquity NanoStation M2. I'll use as a WDS station based on my Cisco WAP121. As a plan B, I'll use it as a second WAP.

Radio propagation pattern, with is horizontal and vertical angle of 60 degrees, should be ideal to send my WiFi signal down through my garage wall, to my car right behind it. I would expect to need only a fraction of the 11dB the NanoStation M2 is capable of, but you never know.