the essentials are a good barrel, an air system and a loader
personally i would suggest a barrel first. you could get a n2 system first, but that wont do much good with a crappy stock barrel. as far as which barrel goes it all depends on how much u have to spend. you could buy a Custom Products One-Piece barrel for $50 bucks and get a good quality barrel for cheap. but if you want something more versatile, go with an insert barrel or adjustable barrel system similar to the Freak or something.
for air system, you will find that nitrogen is a much better propellant than co2. for your needs, a simple preset tank (output is preset to 800-850psi) will be more that good enough for the spyder. personally i suggest PMI 4500psi tanks. but at this point, nearly any preset tank is high enough quality and good enough flow. but what ever u do, spend the extra money on a 4500psi system. do not settle for a 3000psi. its nice to have that extra air, and even if u cant fill to that high, its nice to have the option for later in your pballing days.
the loader is all based on your needs. if your just starting and dont have a feeding problem, use a VL200 or other non-agitated loader. if you want to be sure not to chop, pick up a Revy or Rico and that should be fine. i highly doubt you need an Evo2 or Halo B. that might be a little too much for just a spyder xtra.
well, i just threw alot of stuff at you and i hope that it all made sense to you. if you have any questions pm me or email or something. good luck