I had similar questions - maybe this thread will help you..
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/144427-about-b...-kit-lens.html
Conclusions for me were..
Sigma, tamron, Pentax - whatever you buy, you will be happy as IQ on all 3 is great.
On the Pentax - if you plan to shoot non-macro - the lack of a focus limiter might be an issue for you, but you do gain quick shift with a Pentax which helps..
Don't discount older glass - it is very good..
I went for the 'plastic fantastic' Cosina F3.5 100mm for the following reasons...
£400+ is alot of money for me..
Im new to macro, and everything I have read indicates that it is a completely different kettle of fish to normal photography..
So I figured, get a cheaper lens and learn about macro and how to do it well before making my first ever big lens purchase. The Cosina comes with a very good reputation..
A couple of things to do
Check out the lens reviews on this site - they are really useful
Use flickr, I searched for the cosina and there is a group for that lens and I liked what I saw.. Flickr is great for seeing what other people are doing with the lens that you want to buy..
Remember that one of the reasons we like Pentax is for the ability to use older glass..
Im still going to get a new macro lens in the future as it is the one area that really has my interest - this way I can learn how do it, get an idea of what I want in my new lens - AF/Quick shift/ focal limiter/focal length etc so when I do splash the cash I will be ale to hit the ground running...
Hope this helps