Is there a reason you like that head unit? I'd generally point you at Alpine or Sony, or JVC or Kenwood if you're on a budget.
You only need a line output converter if you keep the factory head unit, and there's not much reason to do that in a 2001 unless you want to keep it stock for some reason.
That's a small sealed subwoofer enclosure. Could you fit something like this? Still sealed, but bigger. https://www.amazon.com/A152-12CP-Sealed-Carpeted-Subwoofer-Enclosure/dp/B0052QDQP8/