The CVT belt generally lasts many of us around 22,000 miles. I did notice on my last belt change that while the belt was still within specs, it had caused my rpms to increase by about 500 over time. I'll generally buy a new belt around the 14,000 mile point to keep on hand and then like the others have said, I'll check it each time I change the final drive oil.
The brake pads are a little different. Again, I'd have parts on hand. Look for deals and just buy them before you need a tire change. The reason I say that is that on my first rear tire change on the 2007 400, the pads looked good...but were wearing unevenly. Having the pads saved me from needing to buy new pads at the garage when they found the pads worn. I'd also recommend physically looking at the brake pads...in other words, pull the caliper off where you can see the pad wear just because of that potential for uneven wear.
So on the 2007 400, my first set of pads were replaced at about 16,000 miles. On the 2008 400...at 26,000 miles, they are only about half worn.