I just wanted to comment that I have a 16.1 APHA gelding and we usually win the 1D at local events and usually end up in the 1D/2D at the bigger things. He doesn't have perfect "quarter horse" confirmation, but he does have very good conformation overall and I have won halter classes with him.
I agree that she does have some faults, but she is also is only 3 years old and will grow some and hopefully fill out some of those faults.
You never know until you try! If you like her personality and think you two will make a good match, I would say go for it.