mmmm. I'm 80kg and, as an inconsistent surfer, would struggle below 40L without adding a lot of outside surfing fitness work plus a few years of experience. As a beginner inconsistent surfer, I can't imagine why your instructor suggested 33L. That's probably a reasonable board once you have surfed for a few years.
The online calculators are a bit all over the place - if you use the Firewire one
http://www.firewiresurfboards.com/volum ... -selector/ it will tell you something around 60L, which seems much more sensible (keep in mind that some makers don't make big mals or mini mals or funboards, so their volume calculator always ends up suggesting volume that happens to coincide with a board they make...)
However, your instructor did see you surf, which I haven't. What were you surfing during your surf camp? How did you find that board - easy, hard, about right?
Most people will suggest getting at least a mini mal (7ft6 or so, give or take) for now and surf that (or even a 9ft and surf that). If you do go somewhere that allows you to surf a lot, then you will know when you are ready to buy a shorter board (if you want to buy a shorter board) and can get it at the time.
However, others will say 'go short now and just work on it'.
I'm in the former camp
As to new or second hand, there is no real benefit in getting a new board if you are looking at a 'starter' board (mini mal) which you then will use to get your skills before transitioning to a shorter board. That said, there are some good quality not too expensive new boards around that will suit - the Torq funboard/mini mal comes to mind. But at the end of the day if the price is much the same as a custom, then get a custom.