Yes, a new member can buy a sold-out resort through Disney as long as there is availability. You still have to buy the minimun 150 points, and there are no incentives for the sold-out resorts, only SSR sporadically.
Also, buying directly through Disney, as soon as you tell them that you want to buy, you are in the system and can immediately make a reservation. The vacation dates just have to be after your closing date.