I booked online with MAV and travelled in the new sleeping car, which was comfortable, smooth, and clean. All per person, berths sold individually, you don't need to fill the whole compartment.
There is a shower, but obviously not intended for use as most of the hose assembly was missing." Fares for Eurostar ICE start at 59.90 each way. (recommended) advance-purchase fares start at just 39 plus either 10 supplement for a couchette in 6-berth or 30 for a bed in a 3-bed sleeper.
Couchettes are fine although fairly basic, a proper bed in the sleeper is more comfortable yet costs very little extra, so go for a sleeper if you can.
There's no restaurant car on between Budapest and the Romanian border so take your own picnic & bottle of wine.
In Bram Stokers Dracula, young lawyer Jonathan Harker travels from London to Transylvania by train.