Windows 7 is approaching End of Life [EoL}, and while win7 will continue to work just fine, lots of folks & biz are expected to move on. Windows 10 will work on a pretty wide range of hardware -- the CPU in my wife's PC runs win10 fine, & it dates to 2011 [she's also got a very recent laptop, so no, I'm not mean] -- but there's bound to be some hardware that won't work with win10, or it may just be old and in need of replacement.
The main difference between buying an off-the-shelf PC & building one is generally the quality of the parts that go inside. The CPU will be the same one you'd buy, but hardly anything else is. Particularly if it comes from a larger company, most everything else will be a cheaper version of what you can buy yourself. Yes, they'll buy stuff in huge qualities, reducing the price they pay, but so will a large retailer like Amazon or Newegg, and you can sometimes get a great deal that's the same or not much more than a PC maker would pay.
What makes it a good time to buy parts is that both AMD & Intel have new CPUs coming, and AMD has new graphics cards too. Anybody anxious to upgrade is waiting until then -- why buy the latest when it won't be in the near future? That means prices on the current generation products are hitting new lows, so now & in the coming couple/few months you can probably snag some really nice bargains.
The most confusing part about building a PC is the number of choices you can make... If you're considering an AMD CPU [they're generally cheaper] you'll need to decide up front if you want to use a separate graphics card, since only a few AMD CPUs include a graphics chip [GPU]. Many more Intel CPUs include that graphics chip OTOH, so a graphics card is something you can optionally add later. As for which CPU to buy, there are loads of review sites with benchmarks comparing lots of different CPUs, and those benchmarks are broken down into different kinds of tasks, as well as different games. Those reviews should also tell you if there's anything else you need to know, like what brands & types of RAM to use, or if you need to buy an aftermarket heat sink or cooler. [Note: I've read of lots of problems with AMD CPUs & RAM [memory], where people simply neglected to look for any sort of mention that whatever brand/model of RAM was designed to work with AMD (?).]
The CPU, and pretty much everything else, gets attached to a motherboard, which is the main circuit board you need for it to run. AMD & Intel each make a couple/few chipsets that go on those boards, the difference being features, performance, & cost -- more of the 1st two means more of the 3rd. The board manufacturers then make several boards with each chipset, the difference again being features, performance, & cost. Note that more expensive motherboards have more retail markup, plus often don't sell as well, so when they're on sale they can be reduced much more than cheaper boards. You normally won't find a big difference in performance between boards using the same chipset, but you will find some brands have Much more support than others, and some boards are best avoided. You can judge the amount of support just by visiting the manufacturer's support site & any official & unofficial user forums. [I personally rate Gigabyte -5 FWIW.] Read user reviews, e.g. at newegg, to get an idea of how common it is to receive a board that's DOA, &/or if there are lots of complaints of the board dying.
You'll likely see stuff about overclocking... Basically it means running a CPU at a higher clock speed than it normally would run at. It used to be a much bigger deal -- you could buy a much cheaper CPU & get the same performance as a higher priced model. Today not so much. If you're a serious gamer into the latest & greatest AAA titles, then overclocking may well be of interest, but otherwise you can pretty much ignore that stuff.
Check user reviews for RAM as well, but generally you can buy based on price, assuming those user reviews don't all say it's trash. You need a power supply, and there are all sorts of calculators online that'll tell you how many watts you'll need. Power supplies work most efficiently a little bit below their max, so you don't want too much overkill -- bigger isn't always better. Power supplies have efficiency ratings, reflecting how much they'll contribute to your electric bill, and many come with cables you plug into the power supply, rather than having everything permanently attached. Some purists like permanently attached cables, because there's no plugs or sockets that can go bad. But you've got to figure out how to cram all those wires inside the PC's case, which can be a very big deal. The power supply market is known for being rife with fraud, so try to find reviews written by a tech who has actually measured output & ideally torn it down. [Once upon a time the weight was a good indicator of a good power supply, so fraudsters started adding rocks -- seriously!] And do be sure to look for some sort of rating, e.g. UL in the US, which is absent a surprising amount of times.
The PC case is often chosen purely for style. How sturdy it is only matters if you're going to be moving it around a lot, or if you plan on using it for years of upgrading [I go 10+ years with one]. More powerful CPU & graphics cards run hotter, so you need to worry more about heat. Your average room temp matters too, since a PC that runs fine in the winter might overheat during hottest summer months. Better cases have more places to mount fans. Some cases are more open, while other may have a sort of wind tunnel effect, with a stream of air moving front to back. If you use an aftermarket CPU heat sink with the fan(s) mounted on the side, the wind tunnel type can be better. Some cases have features to make it easier to add or replace add-on cards &/or hard drives, some let you route many of the cables behind the motherboard, and some are just bigger, giving you more room to get your hands in there to connect stuff. If a small size is more important OTOH, you can go smaller, e.g. a mini-ATX motherboard & case.