With the stock head unit you'll need a converter likehttps://www.amazon.com/PAC-SNI-35-Variable-Line-Converter/dp/B001EAWS3W
Not sure what you decided to go with but IMO if you get a sub in your trunk then your speakers will sound like they're better. Many people on this subreddit will bash this recommendation but I'll give it anyway.
The converter is needed if your radio is stock. I also think it makes the install easier regardless as it is one set less wire to run & you won't have to take apart your headunit.
I think this is the best intro level set up for a subwoofer if you're looking to spend about $220. It will definitely impress you if you haven't heard/had bass in a car before. Also very easy to set up and you can do it in about 3 hours with no experience.
Edit: forgot this amp comes with a built in high/low converter to feed straight off the speakers. Converter may be easier but is optional.
Just my suggestions. Any of these can be swapped based on any kind of preference you have. I left a lot of room because as has been said, with metal you may want to focus more on mid bass. With this build you can try it out and still have left over to upgrade if you feel the need.
If not getting an aftermarket stereo you will need a
I would recommend building your own box. Cheaper and you can typically get better quality materials than a prefab. If not I think this would be an acceptable option. I chose sealed as it will typically give you a more responsive and accurate bass for metal.
EDIT: Most if not all of these items could be picked up on amazon possibly for a bit cheaper. Always keep in mind when purchasing from amazon you may save a little, but you may not have as good of a warranty if at all.