Minimum setup I can think of is some $0:
use Linux (simply drag and drop from your desktop); otherwise:
MySQL + APACHE
use Linux (simply install from a repo); otherwise:
* mariadb is a drop-in replacement for mysql; but you can (of course) still use mysql if you’d like.
Many free options to choose from… grunt, sass, etc., or roll your own.
…I don’t have strong opinions on this.
Everything listed is cross-platform (Linux, Mac, Win) and open source.