Hi Tim!
As far as sturdiness goes, I just don't know yet. I haven't dropped it on the floor, but seeing as it is resin, and glued together, I expect it to maybe loose a few pieces if that happens. It will never be as sturdy as a full metal blaster or a hyperfirm (foam rubber) one. So it may not be the ideal blaster if you're planning on handling it rough (throwing it around and letting it have a beating).
But as I've mentioned before, seeing as it may be the cheapest alternative, along with being one of the most accurate ones too (and a fun build!) you really can't go wrong with one of these kits.
On the functional folding stock: It would very well be feasible on a doopydoos, just replace the resin one with a metal one, add a few bolts and stuff and you're good to go. BUT! If you are going to spend that much time on modding it, you might as well go for a pipe-build, and customizing it even further, maybe add a blastercore (sounds and light), seeing as the doopydoo-kit is (I believe) too narrow inside to fit a soundboard..