Just so you know, check valves don't work very well for SW
plumbing because the salt creep, calcium build-up, etc will basically stop it from working. Sorry!
As for the heater, I don't know of a short 300w heater, so you would probably want 2 smaller ones.
And your sump should always have enough room to handle all the excess water that will drain into it once the return pump is shutoff, regardless of check valves and other items that *may* help. Perhaps an auto-top off would help regulate a lower water level.