Only issue with using OLD Hardware is that your support will be limited. I use to run a 5770 HD prior to my 1070 and had no issues with it. I was able to get ~38-44K h/s for WPA2 on Hashcat 3.20 from my recollection. However, the newest rendition of hashcat may throw errors as it is not OpenCL compliant, so you may need toggle a few parameters to get it to work. Also, using older HCCAP format in older hashcat versions has the possibility of "missing" the correct key just for your knowledge. I have not encounter this issue, but that was the reason for the upgrade to HCCAPX.
Good luck on your venture!
Good luck on your venture!