What scary stories? There is nothing scary about the train, only in the fevered imaginations of those who wish to spread urban myths, misinformation and fear. If you are thinking about gassings and the suchlike, then forget it - such myths have long been debunked as impossible.
This is a civilised part of the world and you can travel in the safe knowledge that you will almost certainly arrive at the other end with your self and your belongings in one piece. I have been travelling around Europe mostly using couchettes as the night accommodation of choice since I was a small child with my family and nothing has ever happened to me, and I don't know of anybody who has had anything untoward happen to them. Put it this way - I would feel a hundred times safer on a sleeper car anywhere in Europe than just walking down the street in downtown Vegas :)
If you want to go in a single compartment you would have to pay a massive surcharge and buy a first class ticket. If you happened to share with somebody in a two-berth compartment the chances are that the other person will be scared stiff of *you*. Realistically, though, that person will just be wanting to travel safely from A to B, just like you. You can even lock the compartment from the inside and nobody will come in.
The sleeper compartments on that train are refurbished Polish cars vagonweb.cz/fotogalerie/PL/PKP_WLABouz.php
A second-class sleeper (2/3 berths) costs from €65. A first class sleeper (identical accommodation, just with only one berth folded down) costs a lot more than that - €193. Personally I would save my money.
Book using www.oebb.at/en - it is the only legit way to buy it online. If you wait until a few days before you will pay €105 for a two-berth or €193 for the single (makes no difference for that one), however you do run the risk that they will have all sold out.