Originally posted by pres589 I'd then go for a Tamron 17-50; it's almost a 16 on Pentax crop on the wide end and has that nice f2.8 constant max aperture. Plus I like the rendering and colors from it.
I would have said the same (except my vote would have gone to the Sigma, since I have and like it as you do your Tamron), but he wouldn't gain aaall that much with it: 17 vs. 18 at the wide end, picture quality and 2.8 aperture from 17 to 35 / between 35 and 50. I'd say with the DA35 and DA50 he's settled for picture quality and fast aperture from ~30 to 50, with the caveat that he has to change lenses, which would not be that comfortable to do while traveling. He would also lose WR / couldn't use the lens as he would his 18-55 now.
I have the 18-135 and 50 myself and I would not say that the 18-135 beats or even matches the 50 for sharpness / picture quality. I myself long for the 16-85 ever since I got my K-S2 with the 18-135 since it's said to be a better lens overall in the overlapping focal length range (except for around 24 or so, where the 18-135 is better?), offers the 16mm at the wide end (but ends at 85 at the long end), but it's bigger, heavier and much more expensive (I haven't seen the 16-85 on the used market around here that much, and if so, it still cost more than the DA limiteds go for). On the other hand the 18-135 is very versatile, a lens that might not leave his camera at all for his trip, and is very affordable used.
A 55-300 would really add something to op's kit, but I don't think it would be such a useful lens for a trip through Europe.
A DA15 or something like a Sigma 10-20 could be an option to expand the wide end of his focal lengths, the 15 being compact, but rather expensive, the other being more versatile, wider, more affordable (at least used), but is not really compact.
All in all I'd say there's no clear recommendation to make here gear-wise, other than to look at all the suggestions and compare them and to penultimately decide for himself what is more important.
Another thing to suggest would be to switch from taking jpegs in AE mode to taking more control of the parameters that affect the photos, and to save them as RAWs to be able to tweak them better to his liking after they have been taken.