Bare minimum? Buy any AP setup the controller on your computer and turn wifi off on your router.
Everything is incremental to that.
Want more coverage? Buy more / better AP’s
Want it to handle routing? Need a USG, an edge router, a dream machine or a dream machine pro
Want to power everything with one device handling POE? You will need a POE switch
Want to offload the controller from your computer? Need a Cloud Key, a UDM, or UDM pro which integrates that ( or use AWS if you want to be fancy / overkill )
Want cameras? Need either a NVR or a UDM pro + HDD
Basically it’s completely scaleable. Buy what you want to do what you want.
Nice thing about the UDM Pro, is it integrates a ton of the stuff that used to be piecemeal into one reasonably price rackmount product.