I happened on this post a bit late, but I'm surprised nobody has built an ATC similar to the Haas design. I suspect the issue is availability of a suitable taper. If you've got a machine with a classic "CNC" taper, it already has a changer.
But consider that even R8's would work. Forget the repeatability aspect and use a tool touchsetter for tool heights and either R8 collet chucks or end mill holders. Ideally you might want to machine these holds with a slot for spring loaded clamps like those shown on Ken Shea's Haas, but even that might not be necessary with a "drop in" design. There, the tool would sit in a well with a through hole large enough for the cutter (and made of something harmless to cutters like plastic) and tapered to keep the holder upright. Using an R8 also eliminates the need to worry about indexing the spindle to line up with the taper dogs.
The v-groove rail that carries the changer is vintage work for the folks on this board--just like a router's Z-axis. Super precision is probably not needed if there is enough give in the clamping sytem--just get the narrow part of the R8 shank into the bore and let it self-align with the taper on the end. That means even an air cylinder might be adequate to bring the changer in and out of position on its rail, another simplification.
That just leaves indexing the proper tools into position. Here I would simply use a stepper and Gecko or other drive. Direct drive ought to be fine, or you could use a belt, whatever is simpler and can manage the weight of a loaded changer.
Mount the whole thing to the mill column and you'd be in business.
Cheers,
BW