'HU' is an acronym for head unit, or in lamens terms your radio.
OK so you want a pair of 12's subs. Since you seem like a real noob, I'm assuming you have no idea what kind of speaker enclosure you want (that would be the box). You have in essence three choices, bandbass, ported & sealed. Since you like heavy metal, I don't think you want super deep rumbling lows. If I were you I'd opt for a sealed enclosure. It'll give you tight, accurate and non over bearing bass. I'd also opt for something smaller than 12's. Personally I love 10" subs.
Since you said earlier that your front speakers need to be replaced, I recommend going with some regular coaxial speakers. Our cars take 6-1/2" speakers in the front. Coaxials are the speakers that have the tweeters built in. The other option would be components which are better but are usually more expensive.