If i purchase an item, like say the melee weapons pack, will that mean those items are unlocked in the game and available to all my characters
Some C-store purchases, like the Melee Weapons Pack is a Character purchase. A single pack only. If you want it on multiple characters, you'll have to buy it multiple times. Other examples are pretty much everything available on the Item list. One buy for one character.
It'll say in the store info if it's a character unlock or an account unlock (like ships, costumes), which are a one time purchase and then avaliable from then on for every character you have or will have.