Bought a new iMac to replace the old, have two questions regarding file migration My old iMac died -- hard drive is on the fritz -- and the system is mostly unusable.
I just received my new iMac and would like to know the proper way to migrate my files from the old iMac to the new one:

Should I create a new Apple ID / login username or use the old?
Given that the old iMac is unusable, what is the best method for transferring my files to the new iMac? I assume iCloud would be best in this scenario since Migration Assistant requires the old system to be operational. FYI, I did not have Time Machine backing up to an external drive.

Thanks!
You want to use the same AppleID as before as both all your purchases and your iCloud Drive is linked to it. So once you've set the Mac up and added your AppleID credentials the content of your iCloud Drive should sync to your Mac (patience may be required, depending on your connection and the load on Apple's servers).