The lightest, most accurate AR was built, not bought, I can guarantee that.
As others have mentioned, if you're only planning to own one, or two, maybe even 3 AR-15's, then you'll never pay back the tools vs. buying a factory model, and will usually even break even by having a guy like Ritch Johnson or Mike Milli (Dtech) build it for you.
Here's a snippet of a standard email I send out to anybody looking to build their first AR, there's more where that came from if you're interested:
Tools you'll need:
AR Specific:[*]Barrel nut wrench (may need proprietary wrench to match your handguard choice)[*]Castle Nut Spanner Wrench (only if you build a Carbine)[*]Upper & Lower Vise Blocks [*]AR receiver Lapping Spindle[*].223Rem Bolt Lapping modified cartridge[/list]
General Tools:[*]Torque Wrench[*]Pin and Roll-pin punches (and roll pin starter punches)[*]Screwdriver set[*]Lapping compound[/list]