Welcome.
Originally posted by serothis I started with the 2 kit lenses (18-55, 55-200) and have since added a 35mm and a 105mm. My most used of those is the 35mm prime, which would ~52mm on FF.
My advice to anyone switching is to consider what you had in the other system and what best matches it in Pentax, bearing in mind that with a shift to full-frame you will be looking at those lenses which give equivalent field of view. Fortunately the Nikon and Pentax crop factors are, if not identical, as close as makes no practical difference. So that makes comparisons easy.
Your Nikon setup translates to 28-300 full frame field of view in zooms, with a 50 and a 150mm prime, roughly speaking. It's been a long time since Pentax made a 150mm prime (manual focus film era), but the FA 135mm f/2.8 is occasionally seen secondhand, and according to the lens roadmap we may or may not be getting a new one next year depending on exactly what "short tele" turns out to mean (some say 85mm because that's what they want, others 135mm for the same reason).
IF you're happy for that 100mm to be a true 100mm and not the 150-ish FOV you had on your Nikon crop camera, the 100 WR Macro (which I own) is good... but it tends to hunt a lot if you miss focus, and it can be noisy. When it gets results, though... excellent (and one user has reported that the K-1's AF system handles this lens a lot better than its predecessors did, so this may have become a non-problem).
Pentax has the 50-ish mm zone covered with the DA50 f/1.8 (not full frame design, but seems to work OK as stated above, and is very cheap for the high quality images it gives), the FA 50mm f/1.4 (film-era but still in production), and the DA* 55mm f/1.4 (more expensive but weather sealed, although some users have reported incomplete full-frame coverage, so watch out).
If I wasn't swimming in primes of various lengths and antiquities up to my eyeballs, all of which I am happy using, I'd be considering the 28-105 as my out-the-door lens when I eventually get the K-1. Unless you go for the older F or FA zooms, some of which are very good (albeit not weather resistant), getting that 28 or rather 24 to 200mm span will cost you A LOT of money. There's reportedly been a patent filed for a 100-300mm lens which would pick up where the 28-105 leaves off, but that's going to be constant aperture (f/4) and likely also big and expensive.
Food for thought. Make of it what you will.