Office build for around $300 help

Solution
If storage space on the individual PCs isn't important than a smaller SSD would be a better choice. A 240/256 GB should have all the space a user would need if they aren't saving files to the individual PC.

Another option for you if you want to make the foot print of each as small as possible.
PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Pentium G4560 3.5GHz Dual-Core Processor ($78.94 @ Amazon)
Memory: G.Skill - Ripjaws Series 8GB (2 x 4GB) DDR4-2400 Memory ($68.99 @ Newegg)
Storage: Hitachi - Travelstar 1TB 2.5" 7200RPM Internal Hard Drive ($54.99 @ Newegg)
Other: ASRock Deskmini 100W ($120.00)
Total: $322.92
Prices include...
If storage space on the individual PCs isn't important than a smaller SSD would be a better choice. A 240/256 GB should have all the space a user would need if they aren't saving files to the individual PC.

Another option for you if you want to make the foot print of each as small as possible.
PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Pentium G4560 3.5GHz Dual-Core Processor ($78.94 @ Amazon)
Memory: G.Skill - Ripjaws Series 8GB (2 x 4GB) DDR4-2400 Memory ($68.99 @ Newegg)
Storage: Hitachi - Travelstar 1TB 2.5" 7200RPM Internal Hard Drive ($54.99 @ Newegg)
Other: ASRock Deskmini 100W ($120.00)
Total: $322.92
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-08-08 18:20 EDT-0400
 
Solution

MadOver

Commendable
Sep 1, 2016
210
1
1,760
For sure you should have a SSD, the rig will never look old, at least loading stuff.

PCPartPicker part list: https://pcpartpicker.com/list/fQYkQV
Price breakdown by merchant: https://pcpartpicker.com/list/fQYkQV/by_merchant/

CPU: Intel - Pentium G4560 3.5GHz Dual-Core Processor ($78.94 @ Amazon)
Motherboard: ASRock - B150M-HDS Micro ATX LGA1151 Motherboard ($50.98 @ Newegg)
Memory: Crucial - 8GB (1 x 8GB) DDR4-2133 Memory ($59.85 @ Amazon)
Storage: Team - L5 LITE 120GB 2.5" Solid State Drive ($50.98 @ Newegg)
Case: Thermaltake - Versa H15 MicroATX Mid Tower Case ($38.88 @ OutletPC)
Power Supply: EVGA - BV 450W 80+ Bronze Certified ATX Power Supply ($21.98 @ Newegg)
Total: $301.61
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-08-08 18:23 EDT-0400
 

PCMACPC

Notable
Aug 5, 2017
282
0
960


That's risky. You took a B150 (Skylake generation) with a G4560 (Kaby Lake), so it could be possible that you need a Bios update.

 

PCMACPC

Notable
Aug 5, 2017
282
0
960
Cheapest but still awesome for your goals:

[PCPartPicker part list](https://pcpartpicker.com/list/ccrNbj) / [Price breakdown by merchant](https://pcpartpicker.com/list/ccrNbj/by_merchant/)

Type|Item|Price
:----|:----|:----
**CPU** | [Intel - Celeron G3900 2.8GHz Dual-Core Processor](https://pcpartpicker.com/product/kxs8TW/intel-cpu-bx80662g3900) | $39.00 @ B&H
**Memory** | [Crucial - 8GB (1 x 8GB) DDR4-2133 Memory](https://pcpartpicker.com/product/xvLypg/crucial-memory-ct8g4dfd8213) | $59.85 @ Amazon
**Storage** | [PNY - CS1311 120GB 2.5" Solid State Drive](https://pcpartpicker.com/product/RzfmP6/pny-internal-hard-drive-ssd7cs1311120rb) | $52.99 @ Best Buy
| *Prices include shipping, taxes, rebates, and discounts* |
| **Total** | **$151.84**
| Generated by [PCPartPicker](http://pcpartpicker.com) 2017-08-08 18:32 EDT-0400 |

Combined this with a Asrock DeskMini 110, and you're good to go.