OK, given I'm not in the US my price sense may be a bit off.
Graphics wise you'd probably be looking at a 1070Ti or 1080 from nVidia, they're ~5% apart performance wise so there's not much difference. AMD wise you'd be looking at Vega 56 or Vega 64 which are similar performance to 1070Ti and 1080 respectively but use more power- avoid Gigabyte and Asus models if possible. Even the cheaper of those options would be close to $400 though, and a fair few would be above. If you need to get a new monitor and have an idea which one you want also consider the type of adaptive sync which goes with each brand; gsync for nvidia but has a fairly hefty price premium, freesync for AMD does not have a price premium. At that price band 4k/60fps would only be achievable on lower graphics games but they should handle 1440p or 1080 fine. I would not bother considering the new nVidia cards as they're all over $400 and the 2070's RTX abilities are... hmm.
CPU wise Intel has shortages and their top end offerings are well above the price performance curve. They still have the absolute performance crown in single threaded applications but you need a Z motherboard, a decent custom cooler and to pay more for the unlocked processor as well. The 8400 is a good mid range option though, as it will work on cheaper motherboards and has a (awful, but 'free') stock cooler. AMD wise 2700X is a pretty good option if you want to do productivity as it overclocks automatically and comes with a good stock cooler, though any of the Ryzens has their pluses and the 2600 will give most of the 2700X's practical performance at half the cost. Gaming wise the CPU doesn't matter that much for mid range builds anyway since nearly everything will be limited by the graphics performance. Note: there are very few ITX Ryzen motherboards, so if you want to use ITX you'll probably be going Intel.
You'd probably want 16GB of Ram despite the price. For Ryzen fast RAM is a decent benefit due to how the chip works and here at least 3200Mhz RAM is similarly priced to 2400. For Intel there's very little benefit.
Get a SSD of whatever size suits you. You probably won't get much practical benefit going NVMe over SATA, but if the price difference is small NVMe may be worth it. Be aware that 'M2' is just a form factor though, you get slower SATA M2 and faster NVMe M2.
There are simply too many motherboards to talk about too much. For Ryzen a good B450 board- eg Tomahawk, MSI's are best this round but avoid their 3 series- is fine even for a 2700X, if you'd use the features of a X470 or think you may want to upgrade later consider them but they are more pricey. Intel wise if you got an unlocked CPU get a Z board, else whatever has the features you want and has reviewed decently.
Case etc: whatever suits. Cheap cases from no name brands will work, but probably have bad airflow etc, an ATX case is probably best in general as it doesn't limit motherboard options. Get a good PSU, they don't cost too much more and actually deliver the wattage they claim at decent efficiency and reliability. Check out a CPU tier list eg and pick one from tier 1 or 2.