The biggest challenge with building the "ultimate" rig from 1989 is that you'll just end up building a computer from 1992 or 1993.
If 1989 is the target, and you want it to FEEL 1980's, you kind of want to avoid the best and just go for the rather excessive.
EGA graphics and monitor.
Some enourmous ESDI monster - MFM era technology but with twice the sectors and a splash of digital. All my ESDI drives are 1988 and 1989 and came with whopping price tags.
Full size AT case with black front plates, your hard drive will need an entire section of this.
5.25" 1.2MB black A drive, with a fancy pants 3.5" drive awkwardly mounted as "B" drive just in case your new game came on one of those silly things
SoundBlaster 2.0 or Adlib sound board (or whatever you fancy really)
Big 386DX motherboard, bonus points if the first 1MB is chip RAM.
Enourmous ISA memory upgrade board - filled with 41256 chips.
It's silly, but I'd find a system like that more fun
Just my view on machine builds though, do as you please!
I remember when I was younger being handed down an old 386 - EGA, twin MFM hard drives (40 and 20), 386SX/16, it was horrible at the time (1994?) but I loved it.