iMac won't Boot with various other problems

Please post then entire KP log.

1. Launch Console.
2. In the Console app sidebar, select the folder named Library/Logs.
3. Select the DiagnosticsReporter folder.
4. Select the most recent crash report to view it. 

Try restarting in Safe Mode per Use safe mode to isolate issues with your Mac - Apple Support

It almost certainly is a hardware failure. The most common failures that cause a KP is RAM and a failing HD. I would recommend uninstalling any upgraded RAM (if there was any) and try to boot with the original RAM that shipped with the computer.

If it starts and runs then you have found the problem. iMacs are VERY sensitive to quality RAM and most users on these forums recommend only using Crucial.com or Macsales.com RAM.

As for a failing HD, you can attempt to boot into Recovery Partition (Command + R on startup), open Disk Utility and run First Aid several times on the HD. If ANY errors appear that confirms a failed or failing HD. If no errors appear that doesn't tell you really anything.

I suspect the next stop for this Mac is taking it to your local Apple Authorized Serviced Provider that has the tools to correctly diagnose the computer and then tell you what the problem(s) are. Then you can decide what to do with it.

I said try an independent AASP, if the computer is considered vintage an Apple Store will not work on it but an independent may.