The K-5 is enough. Much more than enough. For every 100 things wrong with my photos, 99 are my fault and only 1 is the K-5's fault
If you're looking for used lenses, check the PF marketplace. Perhaps a used Pentax DAL 55-300. It has more reach and better image quality than the 200mm lenses you've found. You should be able to find one under $200 USD.
The most important thing is to practice. The 18-55 can give good photos especially when there's enough light to set the aperture (aka "stop down") to f/8.
Keep reading PentaxForums. You will see questions answered before you even knew to ask them.