i went through almost the same thing as you
4inch absolutely beautiful coral beauty, but with ich. i couldnt resist seeing such a beautiful thing die, so i bought it and QTd it....
5months later, its in my DT, fat and healthy. buying or not buying a infected fish isnt going to stop a LFS from not treating them and i also think feeding dead fish to some other marine animal like an anemone is really the right thing to do. I would rather have a marine animal feed another marine animal if it died, like if it would happen in the wild. at least by its death it is helping something else. if i have a fish die, its food for my mantis.
gl on your decision, hope it all works out