Hi and welcome.
K-5 is a great choice. You can find lots of
information about lenses on this forum
Here is a useful table of all K-mount compatible lenses sorted by focal length.
Amazon, BH, Adorama have good prices on lenses quite often. And if you buy from BH and Adorama you also support this forum if you follow the links posted
here Also, if you shop at BH or Adorama you can get Site Supporter status which allows you to sell at the
Marketplace here on this forum
You can find lots of offerings of used lenses there. There is a "Wanted" section as well.
Good luck building your new system.