The base is integral, yes. Though most models are only attached at one point, so removing the base is definitely an option if desired (would require a sharp knife though).
The plastic is a variation of restic, though the mold lines are generally a lot better. Its also a bit more flexible, though not overly so (ie, its not nearly as bendy as Reaper Bones). Since they come preassembled, not being HIPS is a bit less of a problem than it might otherwise be. Unless youre planning on heavily modifying stuff, of course.
(though even then, the hot-water treatment can go a long way)
Hope that helps.