To set a performance target, I suggest starting with a search for midrange gaming builds, then reduce the graphics spec and add more emphasis on memory size, fast storage, and core counts.
Anything leftover in the budget is worth considering for peripherals(monitor and keyboard), ergonomics(monitor stands or arm mounts, chairs, etc., mice alternatives that you can swap to to reduce RSI, macro keypads to ease boilerplate data entry), and subscription services.
More gaming focused builds, but helpful for finding parts that work together if you don't know. If you truly won't do any gaming, then you can get a cheaper graphics card than the build recommends
If you want a real box, buying used is fine. Plenty of companies need top of the line and sell off their old stuff.
Answer these questions first, and that will help you figure out the answers to a lot of other questions along the way.
I have built most of my personal workstations, so I can speak for building your own. I love the granular approach to it - I get to pick an choose across vendors and choose my favorites. Something to take into consideration: building your own *may* be cheaper, but it's not always.
Of course this is valid for workstations - for servers to do target practice I can't stress enough as buying second hand servers is the best options in most cases: you can find servers that were the "hot s**t" 5-6 years ago, for 300-600$.