If you are talking a PC primarily being used for sim racing purposes you do not need anything special.
Go look at the system requirements for iRacing or Assetto Corsa and you will see that pretty much any computer from the last 5-10 years will work just fine.
I picked up a used PC (with an AMD A8-5500) for under $300 that I'm using and it's running Assetto Corsa with ease. That said, I'm not a gamer, so if you plan to use it for other gaming purposes don't bother listening to me
