What sort of tool changer do you want to build?
A rack toolchanger can work well with a gantry-style machine, but not so well on a knee mill, so the choice does depend to an extent on the base machine.
(and very much on if it's a mill or a lathe, you haven't said)
What spindle do you have? It's a lot easier to set up an ATC with a pneumatic drawbar, for example. (If it's any help I converted my threaded drawbar 1960s mill to a pneumatic pull-stud type drawbar, but was lucky that the mill had a fairly generous drawbar bore)
I have seen some fairly clever hacks to change tools on an ER-collet spindle, though, its not impossible.
hackaday.com/2016/06/20/hackaday-prize-e...omatic-tool-changer/
This one rather cleverly uses a circular move to apply more torque to the collet nut than the spindle can provide.
