Welcome bessa. Let me preface the following with I am no god of ground glass and only presenting options.
The current Pentax lineup looks like this:
+ The DA18-55 AL (or AL II) kit lens. It's no slouch. Possibly the best quality "kit" lens in the industry. That's your least expensive option.
+ Next up in price would be the DA 16-45mm f/4.0 which I have not owned but have used and/or handled raw files from to say it renders very nicely.
+ Next comes the DA 17-70 f/4.0. I can not attest to its virtues but haven't seen a lynch mob coming after its designers either. It will run you around $500 and provides you with SDM (silent focus motor) and a bit more range.
+ The DA 18-250mm f/3.5-6.3. This is a "super-zoom" or "utility zoom". It has massive range, and very good image quality if you take into account its inherent limitations. I had a copy and loved it - actually somewhat regret letting it go. There is some barrel distortion at the long end and some light vignetting at the wide end. But it is a
great lens nonetheless.
+ Finally is the DA* 16-50mm 2.8. This is the top-of-the-line short zoom. Better optics, it's fast (f/2.8), SDM AF motor and weather sealed. It comes with a matching sticker price.
For 3rd party offerings there are, for me, two hands down best choices:
Tamron 17-50mm f/2.8 & Tamron 28-75mm f/2.8. I own the Tamron 17-50 and in my opinion as far as image rendering is concerned it's as good as the DA*16-50 and about $300 less expensive. I am finding for my day-to-day shooting I run up against that 50mm mark too often with both my 17-50 and DA*50-135 and am planning to "fix that problem" by adding the Tamron 28-75/2.8 to my bag.
So there are a bunch of options. I'm not going to attempt to sell you one way or the other - they are all good lenses in their own respects. But... keep that old glass too!