Only optional-from Bp maybe advisable-you can also whilst in HU simply buy a domestic seat till Györ or other stop for 300HUF=<1€ from machine-you have your seat all the way.
Note that these ALex trains Prahy-München often get long lasting border checks (mostly for refugees) and may miss their joining/connection via Regensburg-M-plus train will be stormed by the 9€ cheap ticket travellers in DE.
If you cannot find seat and are too shy to ask someone to take luggage away-you stand. Trains make many stops and at all stops people get off and on.