prestige is monthly yes
items on the l2store change frequently. Currently, the expedition pack is decent value for lowbies. But afaik that is only sold for 1 more day, tomorrow something else will be available. Beyond that, at 105, buy as many exp boosts as you can afford. Destiny Pack gor example. If you 're between prestige and destiny, get destiny, prestige is more useful later. The game doesn't really start till 110+, getting to 110 without exp boosts can take 6-8-12 months of you losing money (meaning the cost of consumables is greater than the adena you pickup)
enchant rates are terrible. unless you are willing to spend crazy money on heavenly/grand kain scrolls, aim to always buy what you need outright, not enchant it
at 105, the blessed exalter armor + enhanced exalted fists are fine. Where to invest depends on what you have, and how much you are willing to sped. Obviously if you can afford stage 3 dragon fists, get them, but those realistically will cost you thousands of dollars. Incremental upgrades as you level may be easier to swallow. For example, the aforementioned Expedition pack gives you a set of artifacts, a lvl 5 brooch, a lvl4 red cat's eye, better shirt and bracelet than the free ones you get and a bunch of pots. For 50 eur/usd. If you are willing to buy that, you should be fine for a few levels, you can slowly begin to fill up the remaining artifact slots , find a decent/cheap crit damage ring, and start collecting money for the big ticket items. Elmore cloak (highest enchant you can afford), Bloody Krishna (highest enchant you can afford) will be usefull after 110. From there it's all incremental upgrades. Jewel upgrades (ruby 5, great ruby, same with red cat, etc), artifact upgrades, pretty much every item you will own will at some point have to be upgraded, your first focus should be damage, later you can focus on defense.