18 lamps? Forget JUST an alternator upgrade. Why have that many lights? Getting ready for the zombie apocalypse, only going to use a couple at a time and want spares you can turn on when the other ones burn out, but don't want to risk being eaten by a zombie when you are changing bulbs? There comes a point where you are making so many lumens that you are hindering your ability to see, and imagine the heat.
In that case, instead of buying 800 bucks worth of incandescent lights, buy a couple 10" SR series LED lightbars from rigid. 3200 lumens a piece for a 35 watt draw? Yes please... Roughly $600 bucks for the pair though, ouch. But, 7000 lumens, that would match the light output from 4 off-road lights with 35 watt HIDs or high wattage incandescent in them, considering lumen loss for optics and such, with a lifespan in the 10s of thousands of hours longer. They would also be less susceptible to vibration or rough conditions. You could also add a couple of Rigid's dually lights for another $200 if you find yourself needing a couple thousand more lumens, and you have still saved the money you would have spent on an upgraded alternator, extra battery and wiring. I plan on getting some myself at some point, when I am not busy spending money on other things.
For the rear, be careful with that, it is not legal to use off-road lights on road, and you could pay some hefty fines for flashing someone behind you.
Anyway, look at those trucks that compete in baja races. Even before switching to LEDs, none of them were running that many lights, and for good reason.