V5 has an optional cable to connect to your computer, not to the Internet. It's only useful if you get dropouts from Bluetooth or Ant+. Other than that and the autocalibration, no difference from V4. The Core is less powerful (not noticeable to most) and lacks the carry handle and 11sp cassette.
The older Kickrs, besides being louder, are also not Kickr Climb compatible I believe, and may have more compatibility issues with certain frames, but that's something specific that you'd have to check with any trainer.
Out of warranty used trainers also are pretty risky if you need service on them. I've read of people not being able to source parts from Wahoo.