I built my own one a couple of years ago using this site:
https://www.jib.co.th/web/pcsetspec/main
I wanted something fast, gaming capable, and that would still be good hardware 5 years down the line. I consider a desktop my primary work tool which I rely on daily so the budget was 30k. Went for a Ryzen 5 2600 6 core, ASUS B450 gaming mobo, 16 GB Kingston DDR 4, and Radeon RX 570 4GB GPU.
You should start basing it around the CPU as the mainboard will depend on what processor you choose. For office applications an Intel i5 would be a good start, an i7 will give you more oomph.
Mainboard next and it looks like the Z390 chipset will go with that CPU. Asus, Gigabyte and MSI are all good brands. The memory you've already decided on as a minimum of 16 GB DDR4, Kingston and Corsair are good, a paired set is fine (and allows for the machine to continue running if one stick fails).
I would run your OS and apps on a SSD (250 should be enough space) and have an external drive to store your data and files. This way if (when) Windoze craps itself, you won't lose your data when you format and reinstall.
Graphics cards are not just for gaming so if you're doing graphics intensive work such as video editing or 3D CAD, you'll need a decent GPU. I would go with something like a Nvidia GTX 1650 4GB or similar (I prefer AMD cards as they overclock better). A dedicated graphics card will also allow you to connect multiple screens or a TV, onboard graphics won't.
Many people overlook the power supply on BYO PC, this can lead to problems. Get a decent PSU, minimum 650W, gold-rated ones are better - Thermaltake, Corsair and Coolermaster are good brands. You can calculate how much juice your box will pull here:
http://www.powersupplycalculator.net/ Always add extra for externals such as USB devices.
Case and monitor are purely your own preference and will not affect the performance of the machine. If you're in a hot country make sure the case is loaded with fans. The above hardware also has no bearing on your internet connection/wifi speed, it is all down to the supplier!
If you want genuine software, prepare to keep your wallet open! You can get all the above with a brand named box, but expect to pay 20% extra for that logo on the case and a bunch of branded bloatware installed to slow you down!
This site has some good info:
https://premiumbuilds.com/
And this one for the real hardware geeks:
https://www.tomshardware.com/