Don't do it, it's just not fair to the fish. Maybe you'll upgrade to a bigger tank eventually, but maybe you won't have the space/resources and the fish will be in a tank that's way to small for him. I'm a big fan of tangs too, and my wife really loves the hippo tang's look & personality, but there's no way I'm putting one in my 65g reef, it just wouldn't be right.
There will always be a tang out there if you get the right size tank for it. I say wait until you upgrade, then get the tang.