its seems to be a pretty reliable unit as i havent heard of any failures. im sure some will have buts thats luck of the draw.
mine seems to shift very nicely with 96km on the clock,ive put 25000km since i bought it and its still as good as when i got it.
may pay to check if it has a towball and has been used for towing, that will affect the life of an auto transmission.
seriously tho, 300kms is heaps i'd be more concerned about the engine and service history, and whether the fluids have all been changed regularly.
i personally wouldnt buy that type of car with km's that high unless it was virtually free