1) Orchard Park is forecasted rainy and 68 for next Sunday. I always advise to layer up. It's easy to manage the cold but impossible to manage wet.
2) Stadium lots open 4 hours before the game (9am). If you are planning on parking in a stadium lot, 8:50ish is good enough. Private lots open earlier and can be cheaper depending on where they are. I'd map out where you want to park and look at the best route to get there with traffic, roads closed, etc.
3) if you've never been to Buffalo, I'd suggest the classics. Anchor Bar, Duffs or Gabriel's Gate for wings. Check out Canal side if the weather permits. Local breweries are a fun way to spend the day including Resurgence, Flying Byson, Big Ditch. As far as sights to see, walking around downtown, niagara square and Allentown can be pretty cool.
Enjoy your trip and go bills!