RAM and hard drives are obviously necessary, but usually overlooked. Here are my recommendations.
When you buy RAM, make sure that you find the chips with the lowest latency number. You can find excellent surplus, just make sure that latency is as low as you can find it. And for whatever motherboard you get, max out the amount of RAM it can take.
For hard drives, a solid state hard drive is gonna do wonders for your system's actual response time. For that, buy new. Either a Samsung 860 EVO or a Crucial MX500. Just don't fill it up with data, leave like 20% free. Solid state hard drives are strange that way.