I had also wanted a Powder Blue tang - but the longer I have spent looking at them, and the more I have looked at some other tangs, the more I like some of the others.
One in particular is Acanthurus japonicus - one of the two so called Powder Brown - but that name doesn't do it justice - it is REALLY attractive. It also seems to be more inquisitive and enjoyable to watch. The previous comments about the lack of robustness of the Powder Blue are certainly true. Check out Bob Fenner's comments at
http://www.wetwebmedia.com/GoldRimSs.htm
A 75 is really the minimum size for any tang - and many people would say too small. 3.5 to 4 feet length is said to be minimum.
And even though the Yellow Tang is thought to be boring by some, it has to be one of my favorite fish - seeing a big school of them in Hawaii is unforgettable - and they have a great "personality".
The blue or regal or hippo tang Paracanthurus hepatus is also very striking looking - a bit skittish, like the Powder Blue, but healthier.
Other Zebrasomas are also good - Sailfins etc -
In fact, tangs are fabulous fish - I would love to have a monster tank that I could fill with tangs. Or another plane ticket to Hawaii.
I agree with the Pseudochromis advice - nasty little beggar.
May also want to consider more flow or even a Tunze surge box - I don't have one but they look great.
Good luck.