Pretty sure that none have a private toilet but I could be wrong.
Here's a good site for some initial info that can help you do some further research on the different trains.

Yes, you can buy the 4 berths in a sleeper car for 2 people. We did. In some ways, I kind of felt it's kind of poor form tho. All the people we chatted with in the other cars were really friendly. We're just kind of paranoid about safety/theft on trains, mainly due past research from rail travel in Europe.