Effects that otherwise modify the target's power and toughness, including +1/+1 counters, will still apply no matter when they took effect
(2/1/2014) Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.
To add insult to injury, it'll then keep creatures tapped down with stun counters for the next three turns
However, Sealed differs from Draft in that it is often a bit slower and less focused by virtue of higher variance
In Commander his deck should contain plenty of red cards with high mana values and casting cost reduction, such as Blasphemous Act or Volcanic Salvo - you want cards that you can pitch to get a huge Blazing Shoal pump, but which you also have a good chance to cast normally
That said, Modern is by no means static right now