The only problem with keeping the factory headunit is the power output. The factory unit only puts out about 10watts rms and 30watts max. with this said if you get speakers that require 30watts rms then the speakers alone will not make it louder.
Also sound quality is inferior to aftermarket units. get a decent one for at least $180+.

If you just wanna add an amp look for an amp preferrably 4channel with high level inputs that way you dont need extra convertors or rca's.