Verified answer

If you're getting subs, you'll need an amp.

You don't need one for your speakers, but it definitely makes a difference.

Really I would start with the headunit,thats like the brain of your system. Then upgrading your factory speakers to aftermarket, then a sub/s for the lows. Do some research first, weigh the pros and cons of car audio products and choose a system that fits your taste.

Very important. I would put one to the new door speakers the sound difference is awesome. The amp needed depends on the speakers its pushing. You always want a little bit more amp than you need but not to much. Never go under.