If possible, it is better to buy the hardware separately as mentioned above (if you know how to put a computer together or has a friend who can do that for you). It turns out to be cheaper this way and you can pick a better hardware. In those prepacked deals you will certainly get a not so good PSU (perhaps the most important item on your computer), a subpar custom GPU, memories, etc. The only thing that you know for sure in those prepacked deals is the CPU, the brand of the case and the cooler. Sometimes not even the cooler. I don’t think you can choose the brand of the PSU, the GPU and the memory. You can customize it, but then the price will soar through the roof.
I also don't recommend used hardware (unless you are getting a bargain), because with the new Ryzen series, Intel was forced to upgrade the i5 to the same level of performance of the old i7s (i7-7700K / i7-6700K) and still keeping the i5 price. For example, if you buy an used i7-7700K or an i7-6700K (stock price around $360), you will end up paying the same 200/250$ of an i5-9600K, which is new, has 6 physical cores and has the same performance of the old i7s. And new motherboards like the Z390 UD are cheap and have a beefier VRM than the previous mid tier Z270 / Z370, plus they can handle an i9 in the future.
But ROF and the whole Il-2 series don't require a super-processor and don't use hyper threading, hence why I mentioned the i5-9600K. I have the i5-9600K, and Flying Circus, ROF and the whole Il-2 series won’t break a sweat. On Il-2 with 12 planes, my CPU hovers around 8% usage (I’m overclocked) and with an average temperature of 35ºC with 25ºC room temperature. In ROF, the CPU hovers about 30% usage and get and average temperature of 42ºC.
But you can use the extra cores in new games and rendering. And these CPUs are coming soldered (no more TIM paste), then it is fairly easy to overclock them to 4.8Ghz+ with low temps and with a mid tier cooler like the Maelstrom 240 or any Corsair 240, or an air cooler NH-U14S, the latter requiring motherboards with the GPU on the second slot, like the Z390 UD.
Regarding GPUs, I think the GTX 1660 Ti is enough to run these games in 1080p. In general people use Nvidia cards for ROF, Il-2 and DCS. I have a GTX 1060 6GB and I can run Il-2 Battle Series in High fairly smooth (not 100% smooth), and I have a 2560X1080 monitor, which is bigger than the full HD ones. And the GTX 1660 Ti is a decent upgrade from my GTX 1060. If you have some money to spare and you will get a good 1440 gaming monitor, I recommend the RTX 2060 Super. The Super series came with the chip above, so the RTX 2060 Super has the entry level 2070 chip and the 2070 Super has the entry level 2080 chip, and they are cheaper than the respective 2060 and 2070.
Also check the Ryzen 3000 series. People in general aren't complaining about the lower clocks, but the Intel parts are in general better for these games. The only problem is that with Intel you pay $250 for a 6 core CPU, and with AMD you spend the same money for a 6 core / 12 threads CPU, which might come in handy when streaming or rendering videos and tracks.
But I'm not complaining about my i5-9600K. If I would change anything, I would have gotten an i7-9700K. I personally would only buy AMD if they keep improving in the next generation or two. I still think it is too early to say that they are better than the Intel for gaming, but you spend more with Intel and get less performance in total.
Regarding the PSUs that I mentioned, you cannot go wrong with a mid tier Seasonic 650W. Or you can get a Corsair one, which is OEM and have a good support / RMA (some Corsairs are OEM Seasonics). The Z390 UD has a 8+4 pin connector for the CPU (the extra 4 pins for the i9 or heavy overclock), but most PSUs come with a 8 pin for the CPU. Then you have to buy an extra connector (can be a little tricky if you are outside the US or Western Europe). Or you can pick a model with extra pin connectors.
Wait for other people to give their opinion, or ask about the Ryzen 3000 series on the Il-2 forum.