First find how many houses you'll be taking pictures of and how many pictures you'll need for each house. You could charge hourly however I would recommend making an estimate of how long it would take you to photograph the place, travel costs, and time in photoshop and set a price based on your estimate. Charging hourly can get messy and that way you don't need to worry about how much time you spend in photoshop etc....
You should keep the rights to your photos and then can give them permission to use them.