Yes you will need atleast one amp, and you shouldn't have a problem replacing your existing head unit with the empeg and adding an extra amp or four. The hardest part would be wiring the amp(s) the correct way and making it look good and stay maintenance free though all the tight turns and off-roading you might do :)

Depending on how many, and what types of speakers you want to drive (and the quality of sound you want), you shouldn't have a problem finding a decent amp (price wise). Power might be a problem if you plan on driving large 12's or any strong subwoofer. I have two amps (one under my driver's seat and one under my passenger seat) a Kenwood and a Sony. They work really well, and can't be seen; but they do just barely fit under the seats. Make sure you 1. have a decent place to put the amp(s) and 2. that there is enough air circulation to avoid extreme heat if you are drawing a lot of power. 3. That you have an easy enough way to run the power, ground and lines from behind the dash to the amp(s). I went under my carpeting, so there is no evidence of any amps. Also, try to keep your ground for the amp(s) as close as possible to the amp(s), make sure it is a good ground, and the gauge of the power and ground lines is more than enough. I hate responding to automobile fires.....

TheJourneyM@n