Thanks for all of the information.

I just booked 4 tickets (2nd class with seat reservaions), Budapest to Vienna on the Railjet via the http://www.mav-start.hu website.

1. Selected E-Ticket
2. Registered.
3. Confirmed registration email (which logged me in).
4. Entered Invoicing Data (no CC is required).

Straightforward from there to Select Journey through the rest of the process.

Got the 19 Euro pricing which included seat reservations. ^