Personally, I wouldn't buy a second hand bottle without an inspection/ test. Also, unless it's insanely cheap (btw that would also raise a few worries) I wouldn't buy a bottle older than 5 years old.
Another thing, is the reason you're talking about a 12l bottle that that's what is available. A more modern 7l/300bar bottle wouldn't be much different in terms of number of fills but be smaller and lighter. That aside, I would be extremely careful about buying used without testing irrespective of whether it's in date or not. Please be careful and good luck.