For wattage, think 5 watts per gallon. For a 20-gallon, a 100-watt heater would be good (some would go a bit bigger to be able to use the heater in a bigger tank when upgrading).
People have their favorites, but I like Ebo Jagers and Tronics. You might want to get a heat guard as well. You can get both at
www.bigalsonline.com (the Tronic guard will fit the Ebo-Jager as well).