Just start studying how to build one and start buying components. With AR-15’s you are pretty safe to mix and match. Uppers and lowers aren’t incredibly important. You can buy aero or Noveske or anything in between and be good. If you have a few punches, a vice, and a torque wrench all you really need is an armorers wrench. If you want it to be accurate but durable I’d get a criterion chrome lined barrel. If you want more accuracy you can get a bartlein or rock from craddock precision. In either case get the barrel with a headspaced bolt.