-so with deck building should i just buy the
Magic the Gathering 2015 Core Set Deck Builder's Toolkit, or is it better to just buy MTG 2015 booster packs?
If you are just getting started out and want to play for fun before deciding if you want tithe part of your earnings to the WOTC gods, Deck Builder Toolkits are the best way to go. You get enough basic land to build any deck you want, a bunch of staple commons, and some boosters. I'd also look to the M15 Clash Pack- if you wanted to buy 4 of them, you could assemble a deck that would do reasonably well at a local tournament, but even buying 1 would help you out
-other question, why does Wizard release so many cards each year, it seems like there are a ton of cards that get released each year? its bit overwhelming to someone new like me, are MTG players expected to collect/buy all these new cards yearly?
so most cards released in the expansions aren't worth picking up unless i want to custom build a deck from that series? so how do you know when you should or shouldnt get new MTG cards?
There are 2 main ways to play Magic- Constructed (which is where you build your deck from your collection and play with it), and Limited (where you build a new deck from sealed product). The 2 most popular limited formats are Booster Draft (You get 3 packs, everyone opens a new pack, takes one card, passes the rest of the cards to the left, and then takes the pack passed to them, takes a card, passes the rest, ect until the pack is empty, then you start again with another booster. Then, you build a deck with the cards you drafted and as much basic land as you want) and Sealed Deck (you take 6 boosters, open them up, and make a deck using them and as much basic land as you want).
A majority of commons and uncommons in each set are designed for Limited- because they are the cards that are going to be most frequently seen in the format, and unless the cards lead to playable decks, the format is unplayable.
If you just care about competitive constructed, you can probably figure out exactly what cards you are going to need from the set by looking at the spoiler list and then ignore everything else, while you need to care about everything for limited
so from what series or years could i build a relevant deck with, and how long does a MTG series or expansion stay relevant before being outdated by competition rules or other things?
For constructed, the 2 most popular formats are Standard and Modern. Modern allows for every card printed from 8th Edition (Summer 2003 set) into perpetuity, and does not rotate.
Standard,
currently, allows 2 Blocks worth of sets at a time. The current structure of a block is Fall Expansion, Winter Expansion, Spring Expansion, and Summer Core Set.
Right now, the Theros block, consisting of Theros, Born of the Gods, Journey into Nyx and Magic 2015, and the Khans block, consisting of Khans of Tarkir (as of right now) are legal. Starting next year, this all goes out the window, with Blocks becoming 2 sets instead of 4 sets, and 3 blocks being legal at a time.
In general, cards are legal for about 18 months when they are printed.