Get the lowest and cheapest speed from them and you'll be fine. You'll certainly be good with 100, but at $9.95 for 50mbps, that is a tough price to beat. Start there and work up if it isn't enough.

Edit: Get this Arris 8200 Renewed modem since you already have a router and avoid the rental fees. That plan I found was a "ACP" plan. Lowest I found without that is $25 for 200mbps.