Depending on where the housing is leaking you might not have to replace the whole housing. There are several rubber seals in the various ends of the housing that may be your problem and can be bought seperately, or at least the one that seals where the uppper and lower housing join can be I know for sure. You dont have to drain the coolant when you do this and you really didn't when you did the thermostat either. It is just a good idea to change the coolant every once and awhile and you might as well when you are changing the thermostat. If you are ideed changing the housing you will have to buy everything together for around $100 unfortunately. I just had to do this same thing.
You will end up getting the upper and lower housing with all the seals along with a new coolant temperature sensor and thermostat. Let me know if you just need just the lower portion of the housing and seals as I have that and both a thermostat and coolant temp sensor left over from when I did this.