Built Mustang Transmissions

I take all input, but dont need the bullshit that comes with it. Ill be the first to admit Im not the best driver, or know everything about cars, this is why I ask questions like these. The reason why I was thinking about getting another transmission was because the shop I took it to quoted me 800-1000 bux for labor/parts…I can pick up a full transmission with wayyy less miles for around that price…