If you want to build a new computer, I would advise driving down to your local Microcenter. The people there are up to date on the specifics of computer hardware. Tell them what you're hoping to accomplish and your price range and they're going to try to suggest different options. As for actually assembling your computer, you can watch various Youtube channels like Linus Tech Tips. Turn off the computer, unplug everything, put computer on a table, unscrew and remove cover, shine a light into the computer, unscrew and unplug the GPU, take old GPU out of computer, put new one in, screw in new GPU, plug in everything, screw the cover back on, take computer back to desk and plug it back into the monitor and wall outlet, should work. The only part that you can really botch is replacing the processor, as you need to apply a gooey substance to it (forgot the name of it), and you don't want to spill it on to the other parts of the computer.
The games listed in the OP are not graphically intense by today's standards. The Steam pages recommend using Nvidia 1080 cards, which are so old that IIRC Nvidia doesn't even produce them anymore and are thus pretty cheap. You might be able to acquire one for $100 or under. However, dealing with used graphics cards can be a hassle and they don't come with warranties, so it might be better to just buy a new card. The way PC hardware works is that the very latest stuff costs exorbitant amounts of money (Nvidia 40XX series), while stuff that is just a few years old is dramatically lower in prince (Nvidia 30XX series), and the stuff older than that is relatively cheap (Nvidia 10XX and 20XX series).
If you intend to record gameplay footage, you will want to buy at least a couple terabytes of SSD space. 4k OBS footage accumulates very quickly (1.1 gigabytes per minute for me, a 3 hour recording can take up 210 gigs). Record as .mkv files, not .mp4 files (you can remux the files later into mp4 for easier editing), so if you crash while recording, your file isn't corrupted.