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Are you saying that CarMax is known to be willing to negotiate on price? That would change things...
The advantage of CarMax is financing, but also they will get whatever car is in their inventory, basically anything.
The alternative here is to get a older used car for your downpayment ($2k) and fix your credit, save a bigger down and then get what you want next year.
Look at it this way. The Fed Rate is under 1.0%, the banks want to charge you 10-12%. If you take that loan.... Wow I just wouldn't do it, not on a used car.
You already will have the expense of additional insurance, fuel and maintance cost, at least make it worth your while.