You can do 8 discus in a 55 very comfortably....just that when they become full sized adults (6"+), they will be a tad cramped. So a 75 would be perfect if your doing 8 discus.
Oh, and the best advice I can give you is to not buy from your LFS. Buy HEALTHY, QUALITY discus from a local breeder, or a breeder/importer who can ship to you (NOT somethingphishy.com). They will generally have much higher quality fish available at a better price, plus you can always ask them questions that you may have. Just because its expensive doesnt mean it's quality. Know discus or no discus!
-Ryan