Can't comment on whether they are overpriced or not (but it is a convenient store,.
However, I have just measured the current consumption of an old coolermaster 80mm fan - startup current was about 130mA, the running current was 100mA. As the OP said he needed to run a couple of fans - a 400mA PSU would be more than adequate. A 4.2A PSU would run 40 fans - but I don't think he was planning ahovercraft!!
Given that this is for use in a marine tank, and he is going DIY, I wouldn't recommend 230V fans.