I still run my old core 2 duo e6400 2.1ghz as a htpc, 4gb ram, windows 10, r7 240 (previous 6570 was having issues and the original x1300 isn't compatible with 10). The old intel igpus don't have a windows 10 driver and runs off the basic windows driver so you'd want some gpu that is compatible. This pc also runs world of tanks and some other games like that pretty well on medium. 2gb ram gets eaten up just browsing so I'd suggest 4gb.
I got my 240 for $25 after rebate back in like January but prices have gone up even the month after I bought it up to $40, now it's $50. Depending on how much you want to spend and if the mobo is compatible with uefi gpus, you could just get a cheap 710 for $30 or a 1030 for $70. There isn't much value...