For the 5 star gear how many pieces you get for $99.99 do you get all pieces or just 1?

You only get 1 piece of 5* gear for 99.99

No matter what set you buy, it's 5 pieces, just the more expensive sets like the one I think you are talking about is a 5-star set with much higher bonues that if you tried to upgrade it yourself from say a 3* set, would take forever. Its up to you if the bonuses fit your play style, but the top players are typically spending on sets like those to jump ahead