Two things outside of outright processor or battery failure kill cellphones. 1. No more manufacturer support, 2. The endless operating system updates that eventually fill the onboard storage, and those updates don't get loaded on removable storage like a Micro SD card.
Consider A Samsung or Motorola Droid that's only a version or 2 old. Also know that most newer cell phones no longer have removable batteries.
Wouldn't worry about 5G, more money than you want to spend and the service is nowhere near widespread nationwide yet.