By it being a GT, he should take that price to around 7000 since it has a few problems and 90k miles on it. Also since your dad somewhat knows the guy, it shouldn't be that hard for him to drop the price.
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Exterior- stock painted black wheels, clear corners, stainless steel grill, cross and drilled front and rear rotors with evolution brake pads.
Coming soon- 18 inch black mustang bullet rims, GT/GTP rear bumper, tinted windows.