What size speakers does the car have?
Components are better than 2-way (coaxial) or 3-way (triaxial) speakers. They take more power, get louder, and sound cleaner.
Yes, you need to keep the amp cool. In the trunk is good. You can mount it to some kind of board and put the crossovers there too.
Music covers a wide range of frequencies, from 20 Hz to 20,000. Most main speakers (6.5" and under) will simply not play the lower octaves. Without some kind of subwoofer you will miss out on the low bass. If that's OK with you, then go for it.
1999 Buick Park Ave Ultra
Head Unit: JVC KWHDR720
EQ: Audiocontrol EQL
Front Stage: Hybrid Audio Technologies Imagine 5.25 mids, Morel Maximo tweets
Sub Stage: Boston G3 10", 0.5 cu ft sealed
Amp: PPI Phantom 900.4
Rear Fill: Boston SE953 6X9's