The amount of accessories for AR-15's available these days is almost mind boggling. I would say, stick with a Gov't A2 style lower receiver assembly, buttstock, pistol grip, etc. The upper receiver can be change out in seconds to a totally new configuration, like from Gov't A2 to varmint, from varmint to NRA Match, NRA Match to tactical, etc, just by switch out upper receiver assemblies. Smith Enterprise has a good national match sights for AR-15's. Just about any of the major AR-15 manufactures has a complete list of sights, barrels, forearms, buttstocks, triggers, etc. availabe.

You can search and check on various web sites like Bushmasters, DPMS, Olympic Arms, Rock River Arms, Armalite, etc. and get more detailed info.