PCPartPicker part list / Price breakdown by merchant
| Type | Item | Price |
|---|---|---|
| CPU | Intel Pentium G4400 3.3GHz Dual-Core Processor | $58.99 @ Amazon |
| Motherboard | ASRock Q170M vPro Micro ATX LGA1151 Motherboard | $114.99 @ Amazon |
| Memory | Kingston ValueRAM 8GB (2 x 4GB) DDR4-2133 Memory | $35.54 @ Amazon |
| Storage | Samsung 850 EVO-Series 120GB 2.5" Solid State Drive | $67.89 @ Amazon |
| Case | Thermaltake Versa H25 ATX Mid Tower Case | $33.29 @ Amazon |
| Power Supply | EVGA 430W 80+ Certified ATX Power Supply | $29.99 @ Amazon |
| Optical Drive | Samsung SH-224DB/BEBE DVD/CD Writer | $18.39 @ Amazon |
| Prices include shipping, taxes, rebates, and discounts | ||
| Total | $359.08 | |
| Generated by PCPartPicker 2016-05-16 17:20 EDT-0400 |
G4400 should be just fine for office usage. If don't want to downgarde CPU then you can switch to i5-6500 (better than what you are using) and stay under 500$ before windows.
It has much better SSD and probably more reliable power supply.
I kept Q170 motherboard in case you need the business related features.
I think you can get Windows cheaper for business than through retail, but looks like you have to contact them to find out anything.
https://www.microsoft.com/en-us/licensing/licensing-programs/licensing-programs.aspx