if you're not going to use flash, the DA*16-50 is the best going. you'll have to go into a camera store that stocks them, and try before you buy, because the quality assurance for this lens is bordering on the worst joke in pentax history.
for the same $, you could get two tamron 28-75 2.8s. depending on how tight for room the chapel and other locations are, you may find the range suits you better anyway.
if you could do magic though
get:
DA*16-50 on one K10D
DA*50-135 on another K10D
have a AF540FGZ flash with a sto-fen diffuser on both flashes, on each body
with a battery grip on both bodies too.
that should yield you 1600 shots, enough for any wedding i've ever heard of. expect sore arms later though.