For ATVs and snowmobiles the AGM has the advantage in cold weather. They are also a lot less expensive.
Lithium shines in solar applications and where very deep cycling is wanted without service life suffering as badly , and of they have a weight advantage when used in a battery bank in a motorhome.
For motorcycles, ATVs and Snowmobiles where you want cold weather starting and charging far more than need of reserve capacity, go with AGM or SLA. They're rugged, better in the cold and are far less expensive than Lithium
You can tap into the high beam power by using it to trigger a relay to actually power the light bar from a line to the battery. That would turn the light bar on and off with the high beams and no overload the high beam lights wire and fuse. The only way to tell how powerful a light bar you can use is to know the output of the charging circuit, the demand put on the system by the ignition and light circuits when they are on and a little extra to charge the battery. The ignition would likely only use a couple of amps at most. If the owners manual only lists the demand of the lights and any accessories in watts, you can get the amperage draw by dividing the watts by 12. When you know the amount of draw on the alternator the system itself draws and the output of the alternator then it only requires you to choose a light bar whose draw is a little less than the power available after all normal running demand is satisfied. I hope I didn't overly complicate the explanation of what is really fairly straight forward in doing.