Sounds like a great plan for sure!
Since I am new to the whole Pentax world, but have long worried about the lack of more ruggedly built gear in my price range, I look at the sealing of the DA* series as worth a grand over the life span of the lens all by itself. This way one can drag your gear, as a hobby shooter, pretty much anywhere and know it is not gonna get ruined if dropped in a puddle or whatever mishap comes along in a wet and squishy form.
More important to me is the issue of dust and grit accumulation over the years. I sort of suspect that is the real reason many lenses do not have sealing these days...it keeps consumers dumping lenses after a couple years in order to avoid failure from simple ambient environmental crud buidling up inside the lens.
So, since I was ready to plop down $1100 on a 70-200 f/4IS then I can justify my switch and use that case to get one shorter sealed lens and couple other general use lenses. And later I will add either the DA* 300 or the 60-200 to go with the 50-135...it would be AWESOME if Pentax would add a 100-300 to the DA* line in the next few years. Even if it was $2k I would probably go for it. But I am warming up to the mostly primes idea that I NEVER would have with my Canon gear as the prices are, for the most part, ~2x more.
BTW, thanks for explaining your plan and sharing the results...it helps a lot!!