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Aside from the fact that you said 'decent power' which is completely relative... Buy one, nothing home made is going to give you better power, it won't be cheaper to build it, and if you screw something up it's all your fault =)
Philips and ST Micro make about 120 car radio amplifier ICs. Most datasheets have the complete schematic and a pcb layout.
Most provide 14 Watts per channel at clipping into 4 ohms. They are advertised at 25 Whats per channel and more.