Yes you can use a gold club membership immediately. I would get the membership before filling my cart. Before you get the membership, are you intending to buy $250 in games? If you aren't, the Gold Club won't break even.
... More and more, people around the world are coming to realize that the world is flat!
You should get confirmation of your gold club status almost instantly, including the code (s) to use for your discounts. I'd order the membership first, and then place the game order. Workers and Peasants may be your best bargain there, as you get to double your 1941-42 order of battle for the Red Army..it is a major expansion to Eastern Front.
Winter Soldiers, Invasion of Germany, Black SS, Workers and Peasants, and Fronte Russo are all excellent supplements.
Winter Soldiers includes many scenarios that are small enough to be excellent choices to introduce new players to the game or to play as a first game over Skype. It's the first supplement I purchased and the one I used to learn the system.
Workers and Peasants is mostly large scenarios. The counters are a little thicker than the usual die-cut counters and I like them a lot. I've played a half dozen scenarios and they tended to be very bloody affairs.
Fronte Russo is one that I will play through all the scenarios eventually. Very little armor but generally a lot of space to maneuver.