Breech will be like the omni, I don't think 100% sealed but much better than stock retal. The beauty of this design is not what internals it comes with but how easy it will be to mix and match. Mine will be running artifact breech and 10kg as soon as I get it. Also, an alpha kit being developed for this.
It most definitely comes with internals and mag which they have confirmed is compatible with regular nerf mags. The breech is just like their omni breech being almost air tight full length and stephan compatible. PT is LS sized. I am told it will come with a 6kg spring out of the box. As for parts compatibility, they will be making a whole line of upgrade parts. They are doing a good job coming up with new and useful stuff and deserve lots of support to continue coming up with new ideas. This shell is genius and so versatile.