I would run IM-16 because you generally want to hear it all on the beach. And some small gold, chains, and stainless watches can come in LOW, down in the iron range. So I would dig it all anyway. Yeah, you'll get some junk. But you want all the aluminum junk, otherwise you lose the gold. So dig it all and if you run over something good, you know you'll get it.
As for the sens, you want to back that off from land settings. You'll still get killer depth at 18...I would run it there. And you can try the no threshold thing if you like, but I want my threshold, otherwise you may well miss some deepies or ones that you come close to but don't actually go OVER. That's where the threshold comes in...you hear it go away and you take a second look. As for Smart or Digital, that's up to you. I like digital but who cares when you're digging it all? Fast off, Deep on, Gain at 7, Ferrous tones. Sweep it low, medium speed, dig it all.
Of course your beaches may vary, but we don't have a whole lot of targets here right now and digging it all isn't much of a problem. If you have a lot of fish hooks, iron scrap, stuff like that then you might want to go with IM-12 or -10 or just pay attention to the numbers/tones and decide on what NOT to dig based on conditions at hand.