Best landscape lens for K-5

In my humble opinion you have at least four different options that will let you take the best out of you attempts, from the cheaper up:
DA 18 55 WR "the sealed plug"
DA 18 135 DC "the smart one"
DA 17 - 70 "the unbeliveable"
DA* 16 50 - well, it has a star...

i am unsure wich one between the smart one and the 17 70 is cheaper...

in general terms i would not go for third part lenses as a start, test Pentax colours and smc, you'll never go back!
have fun!