Make sure you have all the terms of the sale in writing and signed by the parties. You may even include an addendum that states that the seller understands and agrees they are selling the property below current market value and that the seller has no expectation of recovering any equity.
Although you've been living in the property, you'll want to have inspections completed. I wouldn't ask for any repairs whatsoever. Inspections are simply for your knowledge.
Check with your tax Assessor's office to see if there are any tax benefits available to you for a family transfer of a property.
You may run into obstacles such the in laws not wanting to sell their home at the price of $580,000 but if everything is writing with everyone in agreeance, it shouldn't be unlike a typical standard sale. Of course, you need to meet your financial criteria and eligibility requirements.
Let me know if i can help