New-to-me 2017 iMac restarting because of a problem

Whenever you acquire a used device, you should always perform a clean install of the OS by first erasing the whole physical drive followed by installing macOS. This way you know you have a clean pristine installation of the OS without any nasty surprises that may be lurking from the previous owner. Plus a clean install of macOS will reveal whether the Mac has a firmware password lock enabled and possibly even whether the device is still being managed by a previous owner. Both are bad and indicate the device was not properly prepared for resale....plus there is nothing you can do about either one so return it for a full refund if that is the case. Some iMacs may have a Fusion Drive, so erasing the Fusion Drive item is the way to go although I personally would break the Fusion Drive and issue the command to recreate the Fusion Drive. Here is an Apple article for what a person should do when selling a Mac:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Even if the previous owner did those steps, I personally would still perform a clean install of macOS myself just to make sure it was done properly and to make sure that the previous owner has not locked or left it in a managed state.

The kernel panic indicates an issue with configuring the power state of the GPU. Hard to say if this is a hardware or possibly software issue since the OS is unknown. If you perform a clean install and still get these Kernel Panics, then it would most likely be due to a hardware issue.

Do you have any audio equipment attached? If so, disconnect it and see if you still get panics. If no other equipment is attached I would return it if you can.

If it works now that you’ve disconnected the cord, seems like it is fine.