With a well insulated shed you shouldnt require a heater in the summer, i mean have you felt the heat lately?
But you inevetabely would in the winter and it would be expensive.
If at all possible try and have the shed attached to the house (just right next to a wall) and if you or anybody in youre circle of friends is handy get the sump inside the house, this would help with heating.
You can also buy 1000w heaters cheap and they are the traditional rod type.
Oh by the way youre looking at 10x4 for one of these monsters, also remember larger volumes of water take longer to cool down.