honestly, you don't need much: an AR armorers wrench and and barrel or upper vise blocks are really the only absolutely necessary tools, though having proper roll pin punches makes it it a lot easier.I don't have the tools to build an AR from loose parts, so I'm thinking of buying some built segments and piecing them together.
you should consider at least building the lower. The 'special' tools are only needed for the upper. some of the 'special' tools for a lower can be substituted with cheaper parts from home depot:
http://www.ar15.com/forums/t_3_4/362116 ... death.html