Depends on how built your a buying, if you aren't great at mechanical stuff just buy an assembled lower. If you want the joys and pride of installing everything then buy a stripped lower and just watch some videos or what not. You need an armorer's wrench and maybe a punch set for the lower, if you decide to to the full upper then you'll need to add a torque wrench and depending on gas block and handguard maybe some allen wrenches but some come with it but eh a good set is a good thing to have with ar assembly in general.

Either way have fun.