My wife just replaced her 2006 24" iMac with a 27" iMac. (My iMac is a year newer and could have Mountain Lion - she was on Lion).
The steps I did were:
Tell her old Mac to not start Mail on boot up, then tell Time Machine to back up now.
Turn off the old Mac, and set up her new Mac.
Ooops, run to the Apple store to buy a Firewire -> Thunderbold convertor.
Restore her old system.
Right click her mail to tell it to start on reboot. Or try. Needed to turn on right click on her mouse.
She can't read the screen - need to change resolution. Tried 1600 x 900 - that looks good for now (My 24" is 1344 x 840 in the main menu and "best for display" in my 2nd monitor)
Adjust Mail to the way she was used to it. Show her how Mountain Lion hides the drag button.
Have her try a full screen game (which she likes to play). It worked.
We recently moved to a retirement community that supplies Wi-Fi (need to occasionally say "accept" to a web screen). My Air Port Express isn't plugged in. I do see her on my computer.
I then opened the App store on my computer and logged into her AppleId. I wanted to know if she had any updates that it didn't see on my computer. The only one was a Java update.
Did I miss any steps I should look at now?