First, the K200D is a very good, capable DSLR. You can use it's pre-set settings, aperture priority mode setting, shutter priority mode setting, or full maual mode. I used one for a long time - 'bout wore it out!!
As to which lens - aaaahhh, the question of the year!! The answer is simple, yet complex. What to you want to shoot? Landscapes, distant wildlife, close-ups of flora and fauna, portraits, sports, etc??
Once you've figured that out, then the fun really begins!! The lens choices can be staggering and confusing. I'd suggest you spend some time reading the lens review section (both Pentax and non-Pentax lenses) and the many posts in the Lens sub-forum. Don't be afraid to ask questions - the folks on this forum, for the most part, are more than willing to answer questions. However, don't just ask "what lens should I buy?" - instead, ask something like "Is the Sigma 70-300mm lens better than the Tamron 70-300mm lens?" BUT, if you use the search function, you'll find that that question has been asked and answered about 87 thousand times!!
Again, by figuring out what you want to shoot will go a long ways to helping you figure out what lens to buy.