Disappearing 9-patch (DP9) is a good one, as is Warm Wishes. Rail Fence is also very easy. I think all would lend themselves to straight-line machine quilting.
If at all possible, get the support group to change the quilt from silent auction to raffle. I personally would never agree to make a quilt for a silent auction. I would make a quilt for a raffle, however. A raffle for a quilt typically brings in a lot more money than a silent auction, making it more worthwhile. In a silent auction, it's quite possible for a beautiful quilt to be auctioned off for less than the cost of the materials used in it. There have been quite a few experiences of this posted on the QB in the past. Raffle quilts tend to fare much better. Typically you sell a raffle ticket for $1, or 6 tickets for $5. It helps if the raffle tickets are also sold leading up to the event. This way the quilt is much more likely to raise $1,000 for the charity.