get the largest battery that you can fit. Who makes it doesn't really matter, there's nothing special to auto lead-acid batteries. The best thing you can do is bring a digital volt meter with you and grab the ones with the highest voltage, preferably 12.6 volts and higher. If they're < 12.50 to 12.55 consider not getting it because it's discharged 25%-50%. Also look at the manufacture date and the ship date! You want to get the latest manufacture date. If it doesn't have a manuf. nor ship date on it then don't get it because there's no telling how old it is. I also recommend putting the new battery on charge properly before installing it to get it back to 100%. Otherwise, if you throw a new battery in your truck that's already discharged 25% or more, you may never recharge it to 100% running the truck, then the battery will sulfate, become weak, and fail earlier than it should.
Don't buy a deep cycle battery, they are not meant to discharge high current rapidly, as in starting a motor or running a plow. Get a regular old starting battery.
unless you're running the plow hydraulics with the motor not running, you shouldn't hurt the battery. You're only drawing heavy for a brief amount of time, even if it is more than the alternator can put out and that energy is coming from the battery, the battery is getting recharged all the time you're not hitting the plow buttons which is like 97% of the time. Would be no different than starting the truck from the batteries point of view.
It's normal for the lights to dim and the voltage gauge to drop a few volts. For a 2004, more than likely your alternator is fine.
What rpms is the engine at when you run the plow? If you're idling at 600-700 rpms then no alternator in the world will put out the power you need. The rpms need to be 1500-2000 or greater for the alt. to put out significant amperage, unless you change the alternator pulley so it spins faster. Factor in the rpms, whether you're running the radio, heat on high, high beams, and any other accessories and that might simply be the problem. Do you know the amperage draw of the plow? I wouldn't expect it to be more than 40 amps. An easy way to check things is cycle your plow while in neutral and holding the engine at 2000-2500 rpms. If you still get a massive voltage drop then it's a weak alternator or batteries or both.
