I don't think they have any internal organs in their "true" form, and there is no mention of a brain. I think that their cognition is more distributed and robust. Also, I suspect that since sentience is conferred by hemalurgic spikes, their intelligence is more spiritually than physically based.
The Kandra learns to form organs, including the eyes, as part of the process of consuming the original body. They can't form hair and must use the original hair from the body. I assume, though this is not specifically stated (or even implied) in any book, that the bodies they form are capable of growing hair; otherwise their hair would start to look pretty shabby over time. However, if they can grow hair it grows at the normal rate, so if you want hair instantly you're stuck with the original stock.
From TenSoon we learn that a Kandra is capable of coming up with organs on its own, even if they are just handed fleshless bones. However, his speed and proficiency at forming bodies was an oddity among Kandra and a product of his ~700 years of experience doing it. Most Kandra would take several hours to form a body that he could form in minutes.
Kandra wearing True Bodies would have to invent their own musculature, and there are references to them using odd materials (like cloth strips) for hair. They would also need lungs and vocal chords to be able to speak. However, we see that a Kandra can absorb and digest food directly, so it needs no digestive organs.
A Kandra can form a body of any size, provided they consume enough food to increase their mass. If Kandra were real, they could bring dinosaurs to life.
If you couldn't tell, I thought the Kandra were one of the most fascinating creatures in the books, so a lot about them stuck with me.