Buy on PC for later?

fatalmoon65​  When you buy content for PC/Mac, your EA account owns that content; it doesn't get assigned to a particular computer.  So you could buy now and have the content waiting for you whenever you were ready.  Additionally, the PC and Mac versions of the game count as one product, so anything you have available on your Mac will also be available in Windows, and vice versa.

I would however suggest waiting for the next Sims 4 sale, as they're very frequent and you'll probably see one before you get the PC.  There's no sense in paying full price when you might get 30-50% off.