Clearly the 18-55mm is considered a starter lens for new shooters. It's an excellent lens, particularly for the price and very good for brighter light shooting.
The 'Limiteds' is a great place to start looking as well as a few of the older lenses.
I'd first consider:
Limited series, 21, 31,35,43,55,70 and 77. You just need to decide on which FL's work for you and choose the lengths. IMO the 31,43,and 77mm are the best of this group.
I'd also look at the FA50mm f1.4 and the FA35 f2
If you have the money and find either an A*85mm f1.4 or FA*85mm f1.4 then just close your eyes, hit the pay button and enjoy. Probably the nicest lens Pentax ever built.
Next I'd give some serious consideration to a few Takumars which are available used. Lenses like the 28mm f3.5, 35mm f3.5 and 105mm f2.8. Not speed demons but very nice glass on the K20D.
For macro theres a number of very good choices, Sigma 105mm f2.8, Tamron 90mm f2.8 or Pentax DFA 100mm f2.8 plus the voitlander 125mm and older Kiron/Vivitar 105mm f2.8
Check out this thread:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/31601-takumar-club.html
Also look at the review databases on the forum. Every Pentax lens and most Asahi variants are there to read reviews, plus many 3rd party lenses as well.
Congrats on the new camera!