The Sigma is a modern (post film era) design, so there's no reason for it not to be safe to use on your K3.
That said, have you given any thought to a flash that not only bounces/tilts, but also has a smaller fill flash? Here's an example by Metz:
Metz 58 AF Digital Flash Gun for Pentax DSLR Cameras XX P17 0082966150326 | eBay
The reason for bounce/tilt is so that you can bounce the light off the ceiling or wall and get really soft lighting. The problem of bouncing of the ceiling is that a shadow is created under the brow and your subject's eyes a bit dead looking. The little built-in flash of the Metz throws in a little of fill light to get rid of the shadows created by the main light. But just as important, it also creates catch light in your subject's eyes making them come alive.
If you haven't won the eBay auction yet, then I'd highly encourage you to let it go and find a flash like the Metz that has that extra flash - it is a significant improvement - I'd wouldn't buy an on-camera flash without it.