Well, first of all which factions are you looking at collecting? Axis? Allies? Sino-Soviet Union? If you plan to be able to supply your friends, then you'll probably want at least two factions.
As a general bit of information, if you're looking to collect either Axis or Allies, start with the Revised Core Set. It has some good units and isn't very expensive, and you'll have a copy of everything you need to start a game. If you're -only- interested in the SSU, however, then you might want to opt out of the Core Set. In this case, you'll want a Terrain Tile Set so that you have the basic essentials, and get yourself a copy of the PDF rules.
Rulebook PDF's are avaliable here: http://www.fantasyfl...eidm=123&esem=4