Full steps to backup and restore using Disk Utility

Hello everyone, I've had a problem where I got the vertical colour lines on screen as well as white screens. All in all I presume the issue is graphics card related, and I'm not willing to pay to have that fixed. Sadly I'm not under Apple Care either.

My plan is to backup the hard drive information and restore it onto a Windows PC that I'll be buying soon enough as replacement for this 6 year old iMac. I use both OSs on the go so I'm adaptable, but I'm qualified and experienced to troubleshoot Windows and not iMac so I needed your help, and the questions are pretty basic actually but I thought it'd be safer to ask online users who are experienced with these procedures with iMac.

First of all, with this issue the only access I have is Disk Utility, so I wanted to restore my Macintosh HDD (1 TB) onto an external 2 TB hard drive after partitioning that external HDD. This process I bet is simply called backup if I'm right.

The only issue I have now is understanding security and access rights. How can I restore this image to Windows after backing it up onto the Ext HDD? Will I have access to every single file? I need to be sure that all files will be accessible later on, that's all. I was browsing using "Open Disk Image" from Disk Utility and I browsed through my user accounts on the HDD with all my files, and some were whitened out, and I bet (might be wrong) that they're locked by security. Will I have access to those files after restoring them, and how?

Additional Info: OS - Yosemite, iMac is 6yrs old, forgot model but don't see relevancy.

Thank you, I appreciate your time! 🙂