Do I need an aftermarket head unit (car stereo) for my amp and subs for my 2008 Ford Escape?

Since it's a newer suv would an aftermarket head unit be worth it? I already got an aux cord plug in.

Not necessary.
A decent line output converter can turn most stock radios into a good source for aftermarket sound systems.

The one caveat is that a lot of modern stock radios use some equalization, especially at high volume, to remove bass from the signal. That is done to prolong the life of stock speakers. Not sure if an '08 escape does that or not.

Nope, unless if you can afford a $300 aftermarket head unit. Since you have a stock head unit, you will need a LOC which is a line output converter.

When most car audio enthusiast buy a new car, they tear out the stock sound system in the car. When they tear out the stock sound system in the car, they put in aftermarket head unit, amplifiers, speakers, subs, etc. That's why you can hear loud bass from several blocks away. When the car is near you, that's when you'll hear the mids and highs. Like the instruments and vocals playing. For the people that are far away from the car, they will only hear the low end which is the bass.

When it comes to adding a subwoofer in your car, the enclosure is the most important factor. The type of box that the sub is inside of will play a big role in output and sound quality. Some subs don't play loud and low enough because they are in the wrong type of enclosure.

A cheap $80 sub getting plenty of power in the right box will sound good. A $200+ sub in the wrong type of box lacking power will sound horrible on high volume.