Generally, you''ll need to use a few inputs in Mach3 to read sensors in the spindle,and maybe 1 or 2 outputs to activate the toolchanger. You'll probably need a solenpid valve or two, and maybe relays to activate them? It depends on your breakout board.
You'll need to write your own M6 macro to control the toolchanger. There's a tutorial video and sample macro on the Machsupport website.
Yes, you can build a rack type tool changer using the simple plastic forks.
A Kw is a Kw. Similar sized spindles should draw the same amount of current.