How to organize a ghost walk?

By all means do so. And hiring a Medium is a good idea if you feel you need to. Make this is like a fun thing but with a slight twist to it of a thrill and a little scarey at the same time. Make sure the buildings are safe for you to enter and find out also what you need to do to set this up as a proper buisness venture. Do some research on the places you have in mind, and please make sure no 1 priority is saftey. Best of luck and have fun.