Purphoros just says 20 creatures = game over, which token decks can easily achieve
Another popular Rebound card with a few bucks is Ephemerate, a one mana instant printed numerous times
Sometimes trading a few cards for a single rare card is worth the trade
Only you can answer this question, but if you are curious about giving grading a try, and are grading for value, the safest bet is to focus on the household names: for baseball, Mickey Mantle, Willie Mays, Hank Aaron, Jackie Robinson, Roberto Clemente, Sandy Koufax, and popular Hall of Famer rookie cards are a good place to start
Keep in mind that it only covers the original four Immersive ex cards released in-game, so you don't have to worry about finding all of the Immersive cards
The cards in each of the two decks are not too complex to master and they allow for different strategies without the need to make any of your move counts