You can see from my signature that I'm stock except for Eibach sway bars: they greatly improve the handling feel, including steering feedback. Welcome to the forum.
Welcome! Does base model mean the 2.0 engine? Intake/exhaust are always good for improving the sound of the car. Probably won't see much in terms of performance gains without a tune.
Congrats on your new 2018 Stinger and welcome to the forum!
There are so many options to modify your Stinger, it really comes down to your own style.
I would highly suggest getting an Oil Catch Can and a Rear Trunk Spoiler as starting off points for the car.