Everyone is lying to you. Assuming you're talking about the iOS Simulator, the MacBook Air will do you just fine.
I'm not exactly sure how I'd proceed to learn the MacOS/iOS internals, I'll have to look into it, but I don't think I need to run a VM, unless I hack the XNU kernel? QEMU does run on MacOS so I guess that's additional 4GB - 6GB of memory for the kernel?