Easy things: controllers. Pretty much anything since there are third-party support and ghetto solutions. Ideally, something that is supported would be anything like Xbox 360 and Xbox One controllers -- I think they're XInput, so anything XInput. There are other resources, but this one came to mind:
http://www.pcgamer.com/the-best-controller-for-pc-gaming/.
CPUs... Yeah... You don't really have a choice because of AMD's decision with their previous generation of CPUs... If you want a good CPU, you're stuck with Intel's i5 and i7 CPUs. If you really, really don't have money, then it's still Intel i3's which is a bit problematic as more games use more than 2 cores. AMD can work at a very low budget like its APUs which offer relatively good graphics since it's a weird hybrid of CPU and GPU -- APUs are what the PS4 and Xbox One use in addition to a GPU --, but its CPUs are pretty much obsolete considering their new generation, AMD Zen, CPUs are about to come out next year. Still, it's not that bad... An i5 6500 runs about 200 USD and if you want to overclock which requires that the CPU is a "K", then the i5 6600K is around 230 USD.
Oh, and try to grab a CPU cooler. Why? Stock coolers tend to suck. They just do. Are they functional? Yes, well... not really for AMD's current CPUs which can run hot -- their APUs are fine... I think --, but they're not really ideal even for basic stuff like not being loud. Grab something like Cooler Master's Hyper Evo 212 which is like 20-30 bucks. Do note that depending on the case you choose, it might not fit, so you will have to look at alternatives which are around the price range.
Memory, RAM, 8GB usually, but 16GB has been affordable, so you could get 1 stick of 8GB and add in another later on. Make sure it's the correct type; don't buy DDR3 for a motherboard that only accepts DDR4. On the GPU end which has its own memory, 4GB is cutting it close now which is why people have been recommending that you don't buy Nvidia's GTX 1060 3GB card which while good, does get bottled necked at times because of its lower VRAM. Why it's not called the GTX 1050 Ti which is an entirely different card about to come out is beyond me... Then again, this is the same generation where AMD's fiasco of having an insufficient power port for their RX 480 -- still a good card despite that problem, not sure if non-reference designs have this problem --, Capcom including a rootkit for SFV's recent, but rollbacked update for PC players, Nvidia not having enough supply for their cards, Samsung, and a bunch of other dumb **** that's likely to continue as it always will.
I don't know what you're wanting for Dark Souls 3, but if you're looking to get maxed settings at 1080p, then an AMD R9 290 / 390 or Nvidia's GTX 970 would do the job:
http://www.techspot.com/review/1162-dark-souls-3-benchmarks/page2.html. Dark Souls 3 has a frame cap, so you won't be getting anywhere above 60 FPS. As stuff gets released periodically, the benchmarks were made before the new AMD RX 400's and Nvidia GTX 1000's. Kind of a rule of thumb is that the next generation would be like this: AMD RX 480 <= R9 390 and Nvidia GTX 1060 <= GTX 970. Both of those GPU's are like 200 to 250 USD compared to past generations which were like 300 USD if I recall correctly. For 60 FPS and generally high settings at 1080p, they would probably do well in any game.
There is a problem where not all games scale well with GPUs. One game could run like absolute **** even with the latest and greatest GPUs like Nvidia's GTX 1080, about to be released? 1080 Ti, and "Titan whatever the hell they're calling it now" -- AMD hasn't released their newest power card yet -- while another runs so well that even something like a Nvidia GTX 950 would be fine. Some of it's depending on the game like open world games tend to be demanding on games while a more or they're really graphics-happy *cough*Crysis*cough* while games that want to target a large audience like DoTA 2, League of Legends, Overwatch, and Street Fighter V won't be as demanding, especially at lower settings -- SFV literally lets you drop to potato-level quality to ensure 60 FPS gameplay. There are always exceptions like Metal Gear Solid V: The Phantom Pain which is optimized very well despite being an open-world game -- it's not heavily populated, so there's that -- and aims for photo-realism or Capcom's Devil May Cry 4, DmC: Devil May Cry, and Dragon's Dogma where both manage to go +120 FPS with moderately-powered systems.
Storage... eh... do I really need to explain? HDDs tend to be cheaper and have higher capacity, but can be larger while SSDs tend to a bit more expensive -- they're becoming much more affordable nowadays --, but are much faster and come in different form factors if you really want them like M.2 which attach to the back of a motherboard. Ideally, having an SSD for a boot drive for your OS and games is nice since stuff loads faster, but as they're not as expensive compared to back then, you could just grab a 500GB SSD and call it a day instead of getting a 240-ishGB SSD and a 1TB HDD.
Cases and motherboards. One, cases will determine motherboard form factors. Makes sense since you can't stuff a big mobo into a small case or a case that isn't made for it -- there are small cases that can actually fit large mobos. From largest to smallest, it's generally ATX, mATX, and mini-ITX. Smaller ones will lose out on stuff like mini-ITX won't have as much RAM slots or PCI-e slots than mATX which won't have as much as ATX. It's not really that much of a problem, though. Cases will support multiple mobo form factors; so a case can support up to all three, another only supports mATX and mini-ITX, and then there are only mini-ITX cases. Generally, cases don't have to be expensive unless you want them to be. You can get a 40-ish USD case that's fine and not a piece of crap or you can shell out upwards to 300 USD for some wicked case. Just go with whatever features, size, or looks you want. Seriously, you can get a case like this that's about 60 USD:
http://www.techspot.com/review/1062-silverstone-raven-rvz02/. Do look for reviews though. HardwareCanucks is a channel that goes over cases:
https://www.youtube.com/user/HardwareCanucks/, if you want a visual aspect to it and not just texts and images.
Stuff to consider about cases, though, is air flow and silence. Good air flow means the components will be able to run cooler (as cool as the ambient temperature). For example, the Raven case I linked is entirely dependent on the CPU cooler and the GPU's cooler to cool itself while more common cases have case fans to help out not to mention being larger inside for air flow, but that being said, there are some cases that are simply restricted no matter how large. Silence, some people want their PCs to run as silent as possible which can result in restricted air flow and higher prices.
On motherboards, specifically, they're separated by platforms and generations; AMD's CPUs won't work on Intel motherboards and Intel's older generations of CPUs might not work on the newer ones. On Intel, specifically, they're also separated by motherboards that allow you to overclock and one's that won't; Z-boards are ones for overclocking while B- and H-boards aren't. So, if you grab an i3 or non-K i5 or i7, then don't get a Z-board, but if you do grab a K i5 or i7, then it might be wise to get a Z-board so you can overclock. AMD never really did this from what I know.
Power supply units. Easiest thing about them is they come in modular, semi-modular, and non... not modular. If they're modular, then that means wires are pluggable, so use what you need which can save space and help with cable management. If they're not, then all the wires are attached which can be a problem for smaller and/or more restrictive cases. Semi-modular sits between of having attachable wires and fixed wires. For wattage, make sure it's at least up to how much power is being drawn which PCPartPicker can show you, but there are calculators out there... I think NewEgg has one. Still, it's better to have some headroom, especially if you're looking to overclock. For efficiency rankings, try to get at least 80+ Bronze. More stuff on PSUs you can probably dig up and it's going to be better explained than by me. Also, PSUs do come in different form factors, but that's generally not going to be a problem; most cases use ATX PSUs.
Other stuff: liquid cooling, sound cards, monitors, VR, optical drives -- external is always an option --, wireless cards, headphones, eh. I'm not going into those. Techquickie:
https://www.youtube.com/user/Techquickie, can help, but more in-depth discussion of those can be found like on LinusTechTips and its forums, PCGamer, PCPartPicker and its forums, TechSpot, Tom's Hardware, etc.
