Dad needed an upgrade, he was running on my old small laptop and it was dying.
These are the parts I built him under $500.
CPU: Intel Pentium G4560 3.5GHz Dual-Core Processor
Motherboard: MSI B250M PRO-VDH Micro ATX LGA1151 Motherboard
Memory: Crucial 8GB (2 x 4GB) DDR4-2133 Memory
Optical Drive: Asus DRW-24D5MT DVD/CD Writer
PSU: FSP 500W OEM 80PLUS Bronze EU Power Supply
Case: Reuse
HDD: Reuse
Total: Under $500
Whilst I was building it, it didn't seem to boot up. For a day, I troubleshoot and couldn't find the issue. Initially, I thought it was the power supply but it wasn't. At first, I plugged the case power led, sound, hdd power leads into the wrong pins. Doh! I first corrected that then the second issue was the PCIe slot side pin was pushed down accidentally as it was installing an invisible graphics card. You can see the picture below highlighted in a red circle. All problems resolved within 2 days.
Old blog post I wrote before, people might find it useful for future reading.