I dunno on the 509s. I ride with a couple of guys that have them and they are always stopping to change them out or de-fog them.
I have the Scott XIs and I have only had to switch them out for a dry pair once. As for KLIM, they use to be top notch but are sliding downhill FAST!
I would get the HMK gear and DO NOT get a full face or a modular helmet, you will end up hating it and buying a motocross style anyways. Snowboard boots do work very well, they are light and waterproof. Do stay away from the metal eyelets though.
Always carry extra gloves with you, invest in a spare fuel can and carry it with you on the sled. You don't need to buy sled brand specific oil for the sled, there are other brands out there that are WAY cheaper that do the same exact job and will not harm your motor. INJEX for example is 15-20 a gallon cheaper than the sled brand oil and does the same thing. I won't hurt the sled at all. Carry dry socks. Prepare yourself to spend the night outdoors, even if you never do. The day will come when you may have to and you won't regret it. Always try to go to a "new" area with somebody that already knows the lay of the land, you will enjoy the ride more that way. Wear layers. Take plenty of food and water. Don't take beer, that is just plain stupid, IMO. Fresh snow, buddies and cameras make us do plenty of stupid chit as it is, you don't need booze to help out. Save that for after the ride. Get a shovel, beacons and avy training. Save for the more expensive avy protection stuff during the off season.
Respect all backcountry users, including the irritating twig fairies that create so many headaches for the snowmobiling sport. Stay out of the wilderness areas even though that untracked powder is awefully tempting.
just a few tips off the top of my head. Have fun and welcome.