I would buy 2 ea. 300 or larger watt heaters. I would then put them on a controller. With a tank that size if you have one that sticks on you will have a mess. Most heaters that I have found stick on, not off.
I use this controller. The single stage. With a chilller you could get a dual stage and the controller will control your chiller and heaters.
I agree with multiple heaters plus a controller. That's basically what I do. I've had too many heaters die over the years to trust them. I usually buy Tronic heaters because they seem to die "off" rather than try to cook the tank.