nothing wrong with using windows 7, im using it, i do own 8.1 should i feel the care to install it, windows 10 has grown on me, but think i could only have it if it came installed like a laptop i purchased months ago before going back to desktop, not to worried about keeping up to date as i dont do anything to make use of dx12 etc, plus compatibility tends to work better on win7, but win 10 is good for linking xbox one and play via computer when its not laggy.
anyways your looking at the haswell gear to keep modern enough and still use windows 7.