You sound like another thread, but I would have replaced the hard drive with another one and kept the original just in case.
If you install Windows 7, you need a partition already set if you don't want to remove the EFI and Recovery partitions. If you don't want Windows 8 at all, remove the Recovery and EFI partitions or even clean the drive with Diskpart and install Windows 7 in that unallocated space.
A Windows 7 DVD may not be usable if the system is set so the CSM (compatibility support module) and/or Secure Boot is not set to use "Other OS".