Update: aight, I'm keeping it. Check out my new next gen audio rig 3000:
I cleaned it out and it seems to work fine. The drive belts seem okay: not dry, but maybe a little loose, although the BPM of the music sounds about right. I paid $50 for it.
I have no bloody idea why I'd need it or would want to keep it. I just wanted to try it out, knowing that the store's policy is "no questions asked" if I return it within 30 days. Also, retro is fun...?🤣
So, it's important to ground it because that's going to remove ground hum if you have it. But beyond that, in order for it to be grounded properly, you should ground it to your head unit or your mixer. If your mixer is grounded, then that's fine, that would work. Failing everything else, yeah, the nearby computer would probably work okay. But ideally, you want it grounded to the piece of equipment you've got it connected to. Incidentally, you'll know immediately if connecting it to the computer was a bad idea because if there's a difference in the ground voltage between your computer and your mixer, you will hear an immediate introduction of a humming noise or buzzing noise into the audio.
Either way, it's not really a safety issue, or shouldn't be, if you don't connect it to anything.