Quote: I am not a professional or anything like that, I just shot family pictures and take pictures when I go on vacations.
First would you recommend K-5 and if yes, what lens would you pair it with. Most of the time on a Canon body I used 24-105 because of its range of motion.
Same boat as far as what I do with my camera. I have a few lenses, an 18-55, a 50-200, a 24, and a 50 are all part of my core set. I probably use the 50-200 the least but only because I have a newborn and most of my shots are indoors.
That said, speaking without the context of switching from a different manufacturer, if I had to trade it all in for one lens, I would probably go with the
SMC Pentax-DA 18-250mm F3.5-6.3 ED AL [IF]
Here's my most amature opinion as to why: it's versatile in range without having to change over lenses all of the time. You can sit and think about the how it's less sharp than a prime all you want, and if that's what your doing you probably should get a prime. I'm just saying if I was only allowed to pick one lens, and only one, I would want one with the largest range and thus suitable for most occasions.
Last time I was out hiking I switched between my 18-55 and 50-200 maybe 8 times within an hour period. Also family, I would guess, probably doesn't want to sit around and wait for you to change over your lens either; if you're not taking candids that is.