Kris,
Your Sigma 17 - 70 is a wonderful lens, and should work perfectly for a wedding. As long as your close enough, and your not dealing with a low light situation.
Even the Pentax Pop-Up flashes work great, as fill in flash. Which might be all that you need.
Please don't tell anyone around my place this, but I love shooting weddings.
They're always different, and you get to meet so many interesting people at them.
If you do a good job you'll get another one out of it.
I got 2 jobs out of my last one, both of which will be a hoot.
Have fun with it, and you'll do Pentax Proud
