The intention of the eq is to create a flat curve if you have peaks and nulls. If you are competing for sq, try to get the sound as flat as possible. If you're trying to make your audio sound good to you, then play with it till you like it. Most people here prefer a bass heavy curve and would...
What kind of music are you listening to? If you played 0db test tones and listen to rock music for example, it's going to be way quieter. I think the bass in rock music will max out at about -6db, but I could be a little off on that number.
If he has the room, I'd look at an sql15 ported with a taramps smart3 amp. The sub is 400 and the amp is 300. You'll have to lower the gains on the amp. That leaves $300 for wiring and building a box. And also loads of headroom of he wants to add another sub in the future.
The 2013 comes with a stock 110a alt. Not sure what the 2016 has. I'm running a crossfire vr1000d for the sub and a taramps 100x4( can't remember the model) bridged. Gains set at 150x2.
I don't know too much about them myself as I won't be doing it till I get my next vehicle, so haven't done too much research. For cheaper options I see a lot of guys go with an agm. I'm planning on lithium. Smaller and lighter, but more expensive. I saw someone talking about "glow" something...
You'll need to spend some money on electrical updates. I have a 2013 sonata and get some pretty bad light dimming running about 1500 watts. If I wasn't planning on selling this soon, I'd be adding an extra battery or bigger alt.
I personally haven't heard any of those 3 subs, so can't make a reconnection on which one, however, if you have the room, get a 15. You obviously want to get loud subs you're looking at 2000-3000w RMS subs. Why not go as big as possible?