18 gauge wire for power lead?

18 gauge wire is usually only used for crossovers or head-units. You will need at least 10 gauge wire for most car audio amplifiers and if you're using multiple amps then you could use as low as 1 gauge. Here is part of an article by Crutchfield.com:

"To calculate the current draw of an amplifier, multiply the number of channels by the RMS watts per channel (a 2 channel amp rated at 300 watts RMS per channel would be 600 watts). Double it to account for amplifier inefficiency (600 watts X 2 = 1200 watts), then divide by the average output Voltage of an alternator, 13.8 volts (1200 divided by 13.8 = 87 amps). Since the average music signal requires about 1/3rd of the average power in a test tone, divide by 3 (87 amps divided by 3 = 29 amps). The result is the amplifier's approximate average current draw."  

Here is a  chart from crutchfield.com also, on what gauge of wire you need for different current draws

Look in the owner's manual under "specifications" and it will tell you the normal current draw.  Failing that, look at the size of the fuse that comes with the equipment.  If there is no fuse, then a rough calculation is take the total watts produced divided by 12 volts.  This is not 100% accurate, but will give you a good ballpark.

Most head units draw 10 amps or less, but some high-power head units can draw 20 or more.  Most sub amps draw between 20 and 200 amps.

Yes 18 gauge is fine for the head-unit (unless its one of alpine's head-units with the v-drive amp).