Most often if anyone is using a trian to reach Budapest its from Vienna and they have a choice OBB or MAV for purchasing the tickets. OBB tends to cost more than MAV for the same ticket.
Other than Austria, when you go to the MAV site to find a ticket, to Munich for instance, the MAV site may not be showing all of the options and the MAV site concentrates on trains they can disount. So for a trip to Germany do check the DB site as well as the MAV site.
Traveling within Hungary if you are over 65 is free but you still need to buy a seat reservation (a couple of dollars).
Always get a seat reservation and buy when its convenient, but dont wait too long ... maybe two week prior. You will hear that trains never fill up and that is generally true but not a fact. You will hear that there are always empty seats, generally true, but also not a fact. The tickets are cheap and if your plans do change, $25 to $50 for an unused ticket was cheap trip insurance.
I have been reading a few threads on people buying tickets from OBB and they arent enjoying it. So I thought I would post how to buy one through MAV, and it isnt hard. But it did get a bit long so i have hung the isntructions in the cloud rather than try and put it here. That link will be good for a week or so, after that PM me if you want to see it.