1) Make a bootable copy of your iMac using Carbon Copy Cloner. Also make a Time Machine backup, just to be on the safe side (if you want).
2) Wipe disk and reinstall macOS (without setting up any account)
That scenario will keep anyone from knowing or getting into any personal files and information.
3) When you get back your iMac, restore with Carbon Copy Cloner. Will take very little time to restore compared to Time Machine.
My suggested method is a little more involved, but should give you peace of mind. Its what I do when I have had to go to the Apple Store.