Welcome to the PNW. You should have a great time in Whistler. If you don't like the prices in Whistler you might consider Squamish instead of Vancouver which will keep you out of the city rat race and put you closer to the slopes. I've never stayed there, others can comment on the night life, but I doubt that it's groovy.
Border crossing. My specialty. If you want to get to Whistler by 8 PM you should leave Seattle by 3:00. I added in extra time for the rush hour which you will be diving into as you leave Seattle. It can be brutal and you have to start south of the city from Sea-Tac airport so you have to battle it all. Everett will be the worst, most likely.
As you approach the border you have a couple of options:
The first is to continue on I-5 to Blaine and take the truck crossing which has its own exit and is well marked. Stay on this highway and head directly north after you cross the border until you get to Highway 1 where you head west. This highway will take you through the Vancouver area and will eventually turn into the Sea to Sky Highway that goes to Whistler.
The second option is to leave I-5 in Bellingham by taking the Meridian St. exit and following this street (Guide Meridian) north to the border, crossing at just north of Lynden, WA. Continue directly north to Highway 1 and follow the route as described above.
Unless you like traffic, don't for any reason go through Vancouver by going across the main border crossing on I-5 and continuing on to Vancouver that way. You will have to drive through the city on surface streets and it's crowded and confusing. I've noticed that Whistler sends people through that way on the maps they publish. It's crazy. Don't do it.
Believe me, it's nearly impossible to get through the Vancouver area without major backups. It's just a part of life. Expect it and become one with it.
Which one is best? Well, the Blaine route is shorter, so if the border isn't too busy it would be quickest. The Lynden crossing is usually much less crowded so on a heavy day you can get by more quickly by going this direction. Sometimes border waits are shown on the big reader board above the I-5 northbound lanes as you enter Bellingham, but usually you won't get it there. You can try http://www.wsdot.wa.gov/traffic/border/
which will tell you about Blaine, but not the truck crossing.
As far as the Sea to Sky construction, there isn't that much going on in January but for information about how it might be check at http://www.seatoskyimprovements.ca/
For terrain, head to the top of the mountain and look around. It's all good but the more challenging stuff is on top. There's so much terrain on those two mountains that you don't need much of a guide. Just point 'em down hill and go.