I recommend spending the $15 on the monthly subscription prestige. It essentially makes the game behave in a way (adena/drops) that makes more sense and allows you to begin building up savings beyond spending on soulshots and other supplies. Yes, without prestige you will struggle. For a new player, I would advise leveling a char to 104/105 and decide if you're enjoying the game in its current state. Then, if the answer is yes, go ahead and spend the $15 on the subscription -- I don't think you'll regret it. As an added bonus, there are other items in the prestige pack that will help you gain adena to put towards improving your character from the daily prestige box.
Of course dumping money into the cash shop is the fastest way to gain adena and improve gear. If you do find yourself open to spending more, I would advise choosing either the 30 day rare accessory pack (I think another person mentioned this and explained it well) or the destiny pack which will further increase your exp/drop rates. If I recall correctly, both of these are in the 20-25 dollar range.