What would be my best GPU option

Solution

Don't buy it. It has weak and non-standard size PSU.
To put gaming graphics card inside of it, you will have to replace the PSU. That also means getting a new PC case.

If you really must have HP-Elite-8200, then get minitower version. You won't have problems with replacing PSU and it will support full height graphics cards.
https://www.amazon.com/HP-Compaq-82...82336&sr=1-5&keywords=HP+Elite+8200+minitower

Don't buy it. It has weak and non-standard size PSU.
To put gaming graphics card inside of it, you will have to replace the PSU. That also means getting a new PC case.

If you really must have HP-Elite-8200, then get minitower version. You won't have problems with replacing PSU and it will support full height graphics cards.
https://www.amazon.com/HP-Compaq-82...82336&sr=1-5&keywords=HP+Elite+8200+minitower
 
Solution
You aren't going to run Overwatch @60FPS with a $120 graphics card. I suggest two options:
GTX 1050 TI ~$155 (https://pcpartpicker.com/product/ttQRsY/msi-geforce-gtx-1050-ti-4gb-video-card-gtx-1050-ti-4g-oc)
GTX 1060 3GB ~$190 (https://pcpartpicker.com/product/pKX2FT/evga-geforce-gtx-1060-3gb-sc-gaming-video-card-03g-p4-6162)
While the 1060 3GB is about $35 more expensive, it is like 70% better overall (http://gpu.userbenchmark.com/Compare/Nvidia-GTX-1060-3GB-vs-Nvidia-GTX-1050-Ti/3646vs3649), and thus will give you much more space and freedom in your settings.
I recommend the 1060 3GB. Trust me it will be a MUCH better long term investment. It can also get at least 80 FPS on max settings, versus the 1050 TI getting at least 60 FPS.
 

This is also true. Get something with a bigger case/ psu