If it was me, I would not bother with a new CPU as that would not make a big difference with running the game on, I would buy a cheap GTX680,GTX780,GTX880,GTX980,GTX1080.I will be soon to acquire a more powerful cpu than I currently have, its characteristics are
AMD Athlon ddr4 ryzen series
AMD ddr4 am4 motherboard
RAM memory ddr4 8GB 3200mhz
AMD Athlon ddr4 series ryzen 3000g processor
320gb hard drive
Integrated Radeon Vega graphics
Would this be enough to play it?
The GTX6xx series is old but it will be cheap and is capable of running the game with high (not ultra) graphics. Check the price of each series (the higher the series the better it is) and pick the one that is in your price range.
If you run your operating system on one HDD and install the game on a second HDD then a mechanical HDD will work fine as the operating system will not be using the second HDD for it's activities. But make sure it's a 7200rpm drive and not the 5400rpm one.
I would not play this game on the lowest setting using the integrated graphics of the CPU, it is just a horrible experience. You can download and try out the game on your current hardware and see if the low graphics is playable for you, as upgrading your CPU will make very little difference.
I said in my earlier reply that I don't recommend AMD graphics cards, but that is your choice not mine. You have the information so you need to make up your own mind on what brand you want to go with.