I don't mean to be a wet blanket, but I don't think it's too crazy of a deal honestly. There is currently 3k cash back or 0% for 60month financing offers on 2012 mustang gt's plus getting a mustang well under MSRP is pretty doable especially during winter. So if you finance a used car for say 7% then you probably would be paying the same in the end if you negotiate a deal right for a brand new one. Plus nothing beats having a new car.
I know the previous owner of this car. I can honestly say he may have floored the car all of 3 times. He drove this thing like an old man. haha It is a great car, with a lot of options and was taken care of very well.
The auto versus manual debate will always be there. The automatic in these cars is a VERY good unit. Becuase of the huge array of people ford wants to sell it to they are set up to not jar people when they shift gears. BUT the dealer can improve the feel of the trans if you want. As can after market tunes. I know a guy that has one and it chirps 3rd hard. This is with an aftermarket tune. It also has a mufflers and drag radials and went high 11s. Just depends on what you want to do.
I bought my car from Beast (John) and he was very helpful and pleasant to work with. I know many guys that have bought cars from him. Everyone was very happy with their experience. New or used John will do his best to help you.