Gotta decide the purpose of the rifle first. That will determine certain parts you'll want. Varmit rifle? Super simple? Tactical? SPR? Once you know you can start to plan.
Minimum is a receiver vise block for upper and lower. Good punch set. Barrel nut wrench (some rail systems have proprietary barrel nuts so buy cautiously). Torque wrench, headspace guage if you go totally from scratch....
Just a few off the top of my head.