I think you computer has a very small Power Supply so any GPU will only be powered from the PCIe Slot. You will need a low power GPU, but that game doesn't have very high system requirements.
I think one of these 2 GPUs will work for you, but I don't know if your case allows for full height or only low profile GPUs.
PCPartPicker Part List
Video Card: Gigabyte GeForce GT 1030 2 GB Low Profile Video Card (£72.83 @ CCL Computers)
Total: £72.83
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-03-30 20:54 BST+0100
PCPartPicker Part List
Video Card: Gigabyte GeForce GTX 1050 2 GB D5 Video Card (£111.98 @ CCL Computers)
Total: £111.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2020-03-30 20:56 BST+0100
The second one is the faster of the two GPUs.