That is pretty unrealistic I must say. Your first stereo system you want 4 18" subs in a brand new car? :p
If you want to get 4 18" subs, you're going to want alot more wattage than that. That amp you posted is $800. You could get an amp with more wattage for less money. You don't have to just pick the most expensive equipment and expect the best. Those subs are 3,000 Watts a peice, so if I were to get those subs, I would probably go with 4 2,000 watt amps.
With all that power, you're going to need a new battery so I would install another OPTIMA battery. They're expensive, but it is a must if you want to run all that power through your vehicle.
Install all of this, especially a bunch of tweeters and mids - your system would rattle the doors off your car.