Risks and mitigation when getting a used iPhone
I intend to immediately wipe the phone by using iTunes to install a fresh iOS, as a clean install.


That should be the only major step required. Do a clean iOS install (or factory reset the device). Make sure to install the latest public version of iOS supported by your model of iPhone.


  ➥ What else should I do to address security concerns?


Installing the latest public iOS release should take care of the security concerns whatsoever. You can further ensure if no 3rd party/unauthorized part replacements have been made. The iOS should be able to detect if any crucial components such as Home button or battery has been replaced by unauthorized means.


  ➥ If this phone had been through a jailbreaking, is there a way for me to tell?


No if the device has been reset to remove the Jailbreak.


  Is there any risk to using a previous jailbroken iPhone after doing a wipe and install of an all-new iOS?


None if you reinstall iOS which effectively removes the jailbreak.