The 2013ish to 2017 GTs are a lot better made than the late model GTs. The automatic transmissions were a ZF design licensed to Ford to build. The new 10-speed is a product of GM and Ford's engineering, and they've had all kinds of problems.

The previous generation 5.0 Coyotes were better quality than the newer ones as well. Avoid pretty much any Ford product after 2018, they're pretty much chasing GM down the rabbit hole of expensive, poor quality vehicles.

A good example is the more recent EcoBoost engines in the Escapes and Bronco Sport. They have poorly designed injectors that can leak fuel and start engine fires. Ford's "fix" is putting a drain in, so gas drains to the pavement and they're putting a sensor in to detect fuel leaks, instead of replacing faulty injectors. They've been building fuel injectors for 40 years, they shouldn't be having this problem. 

Welcome new member!

I agree with @justin-shepherd that that particular car is not Ford's best effort.  Not the best drivetrain to start with, and likely to have been beaten on by the previous owner.

In general, I would look for a car with a naturally-aspirated 6-cylinder (ie, not EcoBoost), but I think those are getting pretty old and will have their own problems.

If a Mustang is what she wants, they you're gonna want to wait til you can find a cherry: 1-owner, low miles, complete service record, and clean pre-purchase inspection.  Don't get in a hurry to buy the first one you see.

Super helpful.  Thanks all!