Since you are talking about getting a 300 gallon tank, I would recommend installing a sump on that beast. With a sump you could get excellent filtration, and you can add a titanium heater with an external thermostat that would be easy to adjust and never seen.
I have also heard the heat units go bad pretty fast with the Eheims. Either do what the others say and get an inline heater with a Jager heater or something comparable with a canister filter if you don't want to run the sump.