Simon, the lens looks like it has been run over by a cargo train!
I would not have paid 300,- EUR for that.
Not the scratches outside worry me - the bent looking front assembly (picture from the side) worry me the most (likely a crash lens).
It is very unlikely, you will get that money back, trying to sell the lens later in this condition.
Additionally potential buyers shy away quickly, when a glass damage (scratches, coating, separation, etc) is eminent (myself included).
I don't touch any second hand lenses, where a glass damage can be seen on inspection (be it a small scratch or otherwise).
I got an absolute mint sample of the latest pre ASPH Cron for 600,- EUR, these sell nowadays for 600-900 EUR, depending on condition.
Your deal doesn't look sweet to me - I would back out, if it is not too late.