Since OSX 10.10 (Yosemite) and onwards are free upgrades and can't be purchased, hackintoshes are not illegal.
There is a Hackintosh forum that has all the compatible parts. Luckily it's at the point where it's basically every intel CPU, every nvidia graphics card, and most motherboards. AMD parts have some issues with Mac.
http://www.tonymacx86.com/building-customac-buyers-guide-october-2015.html
^^^ THIS.
Let them buy a Mac Mini and you can then screw it in to the back of any monitor with a standard 100x100mm vesa mount at the back. Then it's basically an iMac but instead it's only a $500 Mac Mini and a $100~ Monitor.