iMac Startup I/O Error Despite Successful Disk Repair
Disk I/O errors literally translate to input / output errors, which occur when the system is unable to communicate properly with the hard drive.  This is very indicative of a hardware failure, whether it be with the hard drive itself, the logic board or the SATA data cable that runs between the two.  Given the age of your Mac (5~ years) I would assume that the hard drive is to blame.  Disk Utility repairs the volume on the disk at a software level,  often it will report when hard drives have a hardware failure but there are also times when it repairs correctly despite communication errors still being present.  

Apple Hardware Test (AHT) IS pretty good at giving you a solid answer but as you've found out it's no longer on your hard drive, likely lost during an old upgrade or re-install.  You should be able to boot to it using the gray restore discs that came with your iMac.  Discs from another iMac/Mac will not work unless it's identical to yours, these are very hardware specific.  AHT should be on the disc labeled "Applications Install DVD", with a small mention of AHT and its version number.  Holding down D while booting with that disc inserted should boot the Mac to AHT.