Optiplex 9020 SFF Deal at Newegg
$114.50 for an Optiplex with everything you'll need to get up and running. You might need a DP to HDMI adapter or a DP to HDMI cable if your monitor only has HDMI inputs. The cables or adapters cost less than 10 dollars.
$124 is about what you'd pay for just a new Coffee lake quad core i3 CPU which is not much better than the i5-4570 you'll get with the 9020.
https://www.newegg.com/dell-optiplex-9020-business-desktops-workstations/p/1VK-0001-3JVW6
Use the HD4600 graphics first, then add a compatible LP graphics card later on if need be.
Dell Optiplex 9020 - Small Form Factor
RAM: 8GB DDR3
HDD: 500GB
CPU: Intel Core i5-4570 3.2GHz 4th Generation Quad Core
OS: Windows 10 Home / 64 Bit
DVD-RW Drive
Display Ports x 2
Intel LAN RJ-45 Networking Port
USB 3.0: 4 Ports (2 on Front; 2 on Back)
USB 2.0: 6 Ports (2 on Front: 4 on Back)
Audio Ports: 2 on back; Mic and Headphone Port on Front
Includes Power Cable, Keyboard, and Mouse