I've very recently bought (but have not yet received) the 18-135mm WR as my one-lens-does-all kit for inclement weather (I don't anticipate long shots under those circumstances, so something longer had no appeal). I have plenty of other lenses, but only one of them (the 100mm WR macro) plays well in the wet. The 18-135 is probably the lens I would buy for a Pentax camera if I were brand new to Pentax right now and had nothing else.
For someone in your position, I would previously have advocated buying the longest focal range lens available (i.e. the 18-270) plus a small, light, fast prime; but after a recent experience where I had to take photos in the rain and found the 100mm WR too long for some of the best shots, practicality has reared its ugly head.
Of course if you live in a fairly dry place, that consideration may not apply to you. And seeing as you seem to be fond of the wide end, a super tele zoom probably doesn't meet your needs as well as some other lenses of similar size and heft.
Originally posted by Grippy although I sometimes do break out the SP500 for some shots still
Go, Spotmatic, go!