$400-$600? I don't think that would even get you the laptops with good intel integrated chips (Iris pro or whatever it's called). Especially not in CAD I would think
IMO the MBPs aren't really that expensive, it costs roughly the same with a comparable Windows laptop. I don't know about the Air or the new Macbook lineup, I think those ones might be expensive for the amount of power they have. But MBPs are pretty competitive if you have use for those specs.
My Lenovo laptop is $500 (rounded up). i3, Windows 10, touch screen 15", Intel HD 5200. I put in my own SSD and it's pretty good for everyday stuff. No gaming whatsoever though, unless I imagine it's 8-bit indie games, but I can't really say I haven't even bothered to try. I wanted a 13" MBP, but since I'm not going to game on it or do video editing, then I couldn't justify it. The 5200 is a sucky graphics chip.
I would say avoid Dell, because in my experience they kind of suck quality-wise (and that's the reputation they have). I'd stick with Lenovo. Probably their Yoga (consumer) or their Thinkpad line (for corporate use).
Get Windows 10. For one, you don't have to deal with constant nagging to upgrade to Windows 10. And secondly, it's pretty good anyway, just gets a bad rap.
Or build a Sager laptop with xoticpc... It'll probably cost you the same amount as an MBP though.