Night train reservations/connection time Romania/Turkey. Does it sell out fast or can I book upon arrival?

Hello everyone, I'm going to Interrail this summer towards Romania and Turkey. Now I cannot make reservations for (night) trains in that area online. Does anyone know if this trains sell out quite fast or are there enough tickets available so that I can do that upon arrival in Bucuresti (3 days in advance) or is it worth booking reservations via a travel agency?
Also, does anyone happen to know what delays I should take into account between Bucuresti and Budapest? I have now planned a transfer of about 50 minutes in Budapest from night train (D78) to Railjet (RJ66), with which I travel to Rosenheim to end up in the Innsbruck part of the NJ to The Netherlands. Is that feasible or is more extensive planning necessary?

Thanks in advance!