I have had my K-7 and now K-5 COVERED in dust, and they both regularly enjoy showers with me to clean off
I am currently deployed to Afghanistan, and if you need any help understanding the fineness of the dust here, think of baby powder lol.
Check out this picture from a dust storm:
K-5, DA* 50-135
I know you said dust with WR seals. I trust the DA* line 100% - I have been meaning to bury my camera and one of my DA*'s in the fine powder to help market Pentax to the naysayers lol. But I wouldn't trust it with a simply WR lens. I think it is sealed just fine, and my DA 100mm Macro WR hasn't had dust intrude, but that doesn't mean that I don't avoid the situation if I can just in case. Rain with the WR lenses, though? Bring it on baby!
But a lens like the 18-135 WR, for instance (assuming you would get that first and not a DA* lens off the bat) will most DEFINITELY give better protection than the Canikon competition against dust. No doubt in my mind. But I haven't developed enough confidence to say the WR line is as dust resistant. First, it doesn't claim to be, and second, it's mainly designed for water intrusion. Some would argue that water intrusion is a harder monster to battle, though.
Regardless, this is my bottom line after all that: the DA* series are completely sealed, and I really, honestly, would have no problem dumping a bottle of baby powder on a K-5+DA* combo, and then rinsing it off in the shower or under a faucet/hose. It wouldn't be the first time cleaning it that way. Honestly. The WR series, while will offer you significantly greater protection from intrusion than the standard Canikon offering, including dust, it is not to the same caliber of sealing. On a scale of 1-10, if the DA* lens series is a 10 for sealing, and the Canikon's are 0 (select SUPER expensive L and Nikkor line not included), I'd rate the WR as a 7, maybe 8-9 if you are adamant on not zooming too much when it's covered in dust.
So in your situation, I would
most definitely consider the Pentax K-5, or wait a bit for the new K-30 which seems to be EXTREMELY promising. I have never had dust as bad as you are claiming in your camera, sealed lens or not. Remember, the body is amazingly sealed as well, so you don't get any intrusion through the body itself, including through the viewfinder.
I hope that helped.
Also, if you are curious, I have chronicled my journey thus far in Afghanistan pictorially, and here is the latest update, number 9:
https://www.pentaxforums.com/forums/general-talk/190291-update-number-nine-my...eployment.html All the other ones 1-8 can be accessed from there
Respectfully,
Heie