I'm headed out to a four day snowmobiling trip in West Yellowstone in mid January and need to get myself some gear. Here's what I still need to buy, and am hoping for some advice:

Helmet

• I'm looking at a 509 Altitude here, but am open to suggestions.
• I tend to run my helmets a bit tighter, but since they're for motorcycles/cars, I don't have anything under them. My current bike helmet is a medium Shoei RF-1100.
• I think I'll size up one size from my moto gear to accommodate the balaclava, but need some advice on this.

Goggles

• Looking at the 509 Revolver or the 509 Aviator
• Any suggestions here?

Lenses

• I'm thinking two different lenses, but I'm not sure which two
• Need a lot of guidance here. Polarized or not? Photochromatic or not? What colors? I'm at a complete loss on this one.

Balaclava

• I've seen the Klim Arctic recommended
• Thoughts/suggestions?

Base layer

• It's going to be cold as hell, but I'm told by the guys on the trip (who are experienced sledders and have rented from this place before) that the rental gear is always new and very nice.
• They recommended all I need to bring is a decent base layer to be plenty warm, even though it'll be single digits
• Since we're going to be sledding for four days, I'd like to have a few sets
• What are your recommendations for base layers? (Material, brand, type, etc.)

Other Advice

• I'm grateful for any advice you guys can give me here. This is my first real snowmobiling, although I'm fairly experienced on just about everything else with a motor.