You can lay them down in the sump but I would still secure them in the sump at some point since you will want to clean, move equipment around, etc and don't want them banging into things. Keep in mind heaters are notorious for failing and many replace them every year or two even if they are working fine. I personally I would not rely on a used heater.
As for the way to set them up. Many run two heaters since as mentioned heaters are very prone to failure. I strongly recommend a separate temp controller. Set the heaters thermostat to 2-3 degrees warmer than you want to keep the aquarium and then set the temperature controller to the temperature you want to normally operate.
The reason for this is if a heater gets stuck on, the controller will shut it off. If a heater fails to heat, the other will keep the tank warm enough. Some run two temp controllers and two heaters but that is overkill on a small tank without expensive livestock.