try the head loss calculator to calculate the amount of flow you would be getting:
http://www.reefcentral.com/index.php...oss-calculator
300w is probably overkill, but would work fine. I use a 200W on my 180 and a backup titanium connected to my reefkeeper as well, using a temperature probe.
My heaters have always been resting in the tank, with the top out of the water on a 45 degree angle. Never heard of not putting them horizontal. The titanium heater is meant to be horizontal as far as I understand.