I would highly recommend an aftermarket HU as it will have the pre-amp connection needed to connect to a amp. You can use the stock HU with a hi-lo converter, but there is a moderate decrease in sound quality.
If you are thinking of doing the system yourself, check eBay. I picked up all my Alpine gear that way and saved a fortune.
Bridging= tying two amps or two amp channels together for more power
Voice coil= electromagnet in speaker that moves the speaker cone in relation to its magnet
RMS= Root Mean Square or basically the usable wattage/power of a amp
More definitions
here.