im guessing the stock system is running 2ohm. how many watt does it say your car came with when you bought it. youll need to stick within an amp in that power range. and youll be looking for a 4 channel and a 2 channel for the subs. a 5channel. 4channel for in the car and then 1 larger channel for the subs. a 6 channel etc etc.
but the question is what are you building it for and how much you willing to spend to get it there. Price will be a very large factor on what the answer is to the question you asked.