Depending on the parts used, you need a torque wrench, a good bench vise with pads, a receiver clamp and magwell support to hold the receivers while working, a punch set and brass hammer, a center punch (to stake the castle nut) and an armorer's wrench (barrel nut, muzzle device) plus a screwdriver for the pistol grip screw (or a hex wrench depending on hardware). That's it for a basic build - and you're looking at around $100 in tools if you already have a torque wrench and bench vise. Proprietary rails usually come with their own barrel nut wrenches. Obviously if you're looking at something like trying to pin on an FSB - yeah, you should pay someone for that. Otherwise, YouTube is filled with videos on how to put stuff together. The AR15 is a very easy platform to work on with those basic tools.