In the late 1990s, Professor Daniel J. Bernstein maintained on his web pages what he called “the standard workstation”, but as the early 2000s wore on, he no longer updated the specifications. This has stuck in my mind as a good concept.

Here is my equipment list for 2020. I don’t have all of it, but most of it is easily available. All of it just reflects my personal tastes and experience; you are not expected to agree with any of it 🙂

Keyboard Lenovo ThinkPad Compact USB Keyboard with TrackPoint
Mouse Integrated into the previous item. 🙂
Display Dell UP3017
Graphics NVIDIA GM107GL aka Quadro K1200
Disk Samsung 860 PRO SSD
CPU Intel i7 or Xeon @ 3.4 GHz
RAM As much as you can afford that will fit.

All in all, the best option for my use right now seems to me to be the HP Z240 SFF (small form-factor) workstation. Although they are no longer manufactured by HP, they are available to buy online at many places. Even better, HP are still producing BIOS updates for them.