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.