I concur with my fellow distinguished posters. It is hardly a conservative choice to purchase any used electronics. There really is a question of mileage, but it is fair to say ones tolerance for risk could justify the savings. You could make off like a bandit, or be taken by one. Either way you have made a choice. Now for your questions.
1. Purchase a used or third party power brick for forty or less. This only has a single use anyway, and frankly since the console is refurbished your not going to invalidate a warranty. Regardless power supplies for consoles are usually rather robust. I have only ever had power supply issues with a console, and that was a first generation Genesis. Continue to save money.
2. A Ethernet cable, a cable modem, and a little prayer. Honestly I doubt your console is banned. Just probably a owner buying up to a newer model. Microsoft loves to purge modded consoles in mass. That said your reasonably far from the mass banning, and with a thirty day return policy its doubtful the seller would make such a offer if the console was banned. You will also need to purchase a subscription for online play.
3. You need one for a few games, downloaded game content, movies, demos, and larger games purchased from the marketplace. Given that Microsoft is selling 20gb hard drives for twenty dollars its worth having one. At that price it is even more cost effective that a card. A five hundred meg card runs upwards of fifty dollars. The drive is not needed the system has ample storage for game saves, but it does open up a lot of worthwhile features that will pay for the drive. Even if you just use the free Netflix month subscription in a gold account.
