Tivo Purchase Advice

I have a couple of friends, married, just cut the cord. They have OTA and a Roku, want to know what kind of Tivo to get. They're not massive tv watchers, just one tv, so they don't need anything too elaborate. They'd prefer to buy used or refurb, if possible. I have a Series 3 HD model, but that seems kind of old, I'm thinking they'd be better off with a later model, but I'm not really familiar with other models.

Thanks much!