It sounds like you have everything you need there to make a nice little boiler. You'll need to remove the built in thermostat for sure. If you wanted to control it manually you could just replace the thermostat with a switch, but you'd do well to consider installing a PID controller for temperature control...they are not expensive.
Before you attempt any of this however, please do your research and learn about wiring, and specifically how appliances like this work. If you read up on how electric hot water heaters are wired for example, you should learn what you need to know.
Proceed with caution. Stay safe and have fun!