You can buy a line converter or use an amp that has one built in. I'm not sure which brand of converter to use, I used a cheap one. Either way will work, I've sucessfully wired it both ways on mine.

As far as the HU, there is a kit to make it possible to install an off-the-shelf HU. (Search) Also, my bro is a Honda tech, and he tells me that it's a pain to remove the factory unit.

After using a sub for a while, I found it difficult to get balanced sound. The sub kept drowning out the rest of the system when I'd get it hitting hard enough. (of course I can't hear much over 13k hz, thanks to too much rock music) I'd say either leave it completely stock, or upgrade the whole system front to back.