I actually have the SFX, which is the same camera as the SF1 (
https://www.pentaxforums.com/forums/pentax-autofocus-film-slr-camera-reviews/...x-sfx-sf1.html) and I shot some pictures with the 18-55 kit lens. They came out sharper than the ones taken with the lens I got with it (Pentax-F 35-80 f4-5.6).
As soon as you mount it you'll notice the black corners in the viewfinder when the lens is at 18. At 24 you don't notice them anymore but I'm not sure if they'll appear in the photos or not.
I'd say you're safe to use it between 35 and 55.
Of course you can try shooting below 35 too. At least for the fun of it. You'll see that at 18 it has a fish-eye look to it. I took some pictures at 18 at a concert where the background was pretty much black so the vignetting wasn't obvious. I haven't finished that roll yet but I hope it will look cool
Like Graham said, the kit lens will only allow you to shoot in one of the Auto modes, Program or Shutter priority mode. No Manual or Aperture priority mode because these newer lenses are missing the aperture ring.
This is a pretty cool camera. I like the 80's look of it. I guess the only downside (besides not being able to control the aperture from the body) I can find is the 1/2000 maximum shutter speed. Not that it's too slow, but the SF1n/SFXn had 1/4000.
Enjoy your new camera and don't be afraid to experiment with other lenses. The worst thing you can do is ruin a few exposures. Film is still pretty cheap
Andrei