The easiest way for your wife to move into her new computer using the account that is on your iMac is this: If you haven't been using TimeMachine to back up your iMac now is the time to purchase an external hard drive (a 1 TB drive will do). See this Apple article to set up the drive. It doesn't matter which account you log into, TM will back up the entire computer.
Once TM has finished, connect it to her new computer. If you are starting the new computer for the first time, go thru the startup and login process until you are asked if you want to transfer data to the computer. Yes you do. You want to use TimeMachine. You'll see a list of accounts you can transfer. Choose hers. You'll also see the application folder. I recommend that you do not select it. Manually download/install applications to her computer. If you've already set up her computer look for the Migration Assistant program in the Utilities folder of the Applications folder. You'll go thru the same steps as mentioned above. Choose her account but not Applications.
Using the TimeMachine backup is much easier and reliable than trying to set up a computer to computer transfer. This is something I tested years ago and I've subsequently tested it with each new OS upgrade and with newer hardware.
Your wife has an AppleID and password she used to set up iCloud on your iMac. She'll use that same AppleID and password to set up iCloud on her new computer. This will sync her mail and other iCloud data to her new computer.
