Adam,
Welcome to PR....Great to have you here..
If you scroll through this pentax section of PR you will see many samples of Images I have shot...In the Pentax world, there are 4 lenses that are a must. I will put them in the order of how I would get them if ''I'' where starting over.

1) Tamron 28-74 F/2.8..This lens is a total God send for the Pentax folks.
2) Sigma 70-300 APO F/4-5.6
3) Pentax DA*50-135 F/2.8
4) Pentax FA 50 F/1.4
5) Sigma 10-20 F/4.0-4.5

Now of course these are for my likes and shooting styles, but these lenses are very good....That K20D rocks by the way...My Daughter has a D200 and a D300 and frankly, I rate the K20D much higher than the D200 and equal to the D300