I'm also trying to buy a Night train ticket from Brasov to Budapest but I really couldn't finish it... it's really difficult to understand how to buy tickets online.
Unfortunately situation didn't change much compared to what it used to be 2 years ago - it is still impossible to purchase international tickets via
Romanian Railways (CFR) official website. Obviously unless you willing to risk & overpay using some agency's services.
I'm planning my travel to mid August and I'd like to know if I can have problems buying the ticket only two or three days before the journey in Romania.
Doubt that there will be any problems buying ticket 2-3 days in advance (NB: remember that you can purchase such ticket not only in Brasov, but basically any Romanian ticket counter). There are 4 overnight trains from Brasov to Budapest, so it's highly unlikely that all of them will be sold out, especially on a week day. The only trouble is that 2-3 days in advance all the discounted promo fare tickets (start from only 19 euro) would probably be sold out, so you'll have to pay standard fare ticket price (which is almost 50 euro for the ticket + seat/berth reservation)
Can you tell me if this travel is comfortable?
Well, it depends on your idea of comfortable (e.g. depending on the train you can expect sleepers look something like
this, seat compartments like
this or
this), but in general international trains in that part of Europe usually tend to have newer/more comfortable carriages than domestic ones