For one, I too live in Houston so I know the drill on the heat...and when it comes to hard freezes, they are rare and we usually have time to plan for them.
As for an opinion, the temp that matters if your equipment is going to freeze is where your pipes/equipment are exposed. I would suggest there is a temp delta btw outside and inside when it comes to colder temp's. It may never reach freezing inside your shed when it hits 34-33-32-31-30 outside, so you would not need to run your pump for freeze protection. Since your not installing a [gas] heater [whole new thread if you were], you could close the vents during cold spells to even add better protection against the cold.
Next, I'd make sure you have more than adequate ventilation for summer bc it gets hot here, esp if your shed will be in the sun all day. Install some vents at top too facing the S and N to provide better drawing of heat...you don't want your pump over heating.
Finally, good point on what you do w the water in the bottom of the cartridge system and how you drain it. I'm not sure if this is available for your cartridge drain plug, but I would check w some good plumbing shops and see if they have an adapter to a ball-valve, then run some flex pipe from the ball-valve to the outside of your shed. You're going to want to drain that water outside and direct it away from your equipment. A brass ball-valve would easily be able to hold any pump PSI...you're also going to want to rinse out the bottom of your bottom cartridge system and this too would re-direct the extra water outside.
good luck,
tstex
