There are also companies that can do mold testing with infrared type meters that will let you know definitively if the mold is gone. The good thing about mold is that once you remove the source (water) it generally will stop growing - but you still have to make sure all the mold is removed to limit any exposure to mold spores in the air. There will always be some forms of mold in the air, but it's understanding what levels and types are appropriate. I would find a mold expert in your area to help you.
You need to find out what caused that mold problem and what measures were taken to rectify it before you can make any good decision. Ask for reports and talk to the company that did it. If you are still interested in the house then you need to get a home inspector in to check it out esp for mold and see if it may still be a potential problem or not. You never know it may be a great house ....good luck.