There are a lot of issues, the main one is the discounted price. No lender would want to do that as they know that your Dad has an interest in the property. There is just no way of getting round that.
You can buy it off your Dad, but only at the market value, though you might be able to shave a few grand off it.
I'm not necessarily sure that this is right, but you will want evidence in writing that your dad does not have any ongoing interest in the property.