How big the chuck on the MIll/Lathe head you aim at? That will be very important at deciding on all elements.
I think most of high speed work can be done with a 5c collet so the rotational load will be low. any chuck work will slow indexing and milling ops. I plan to make the lathe spindle as well so the through bore and bearings can be optimised for the final design.

Sliding head means you'll have quite a bit of weight to move. It's not a major issue, but you need to consider you'll need a relatively substantial bed to support it.
One benefit of going 5C, is a pneumatic/mechanical collet closer setup is pretty simple. You don't need anywhere near the same force as a standard chuck, but you'll need to make sure the spindle front bearing can handle the extra thrust force (or use some other kind of setup that avoids loading the spindle bearings)
Yes I was a bit concerned but I think it will be less mass than the milling column and rotating spindle. pneumatic collet closer is ideal and the spindle will be designed to have as much of the collet as possible in side the front bearings.

BT30 ATC spindle is fairly simple. ATC is the more challenging part, but not that difficult. More time consuming to design and get all the parts needed, rather than difficult.
I'm working on making a power drawbar and ATC for the nova mill so there will be some overlap in projects. Although the rotating head will make it easier to interface with a rotary carousel.

Lathe ATC. Doable. Plenty designs around.
Got that bit solved as we have already made one for a Denford Orac that works very well. I will probably have to modify the denford style tool plate to take 8 turning tools and bolt on boring attachments.


The puller and tail stock are all nice to have but not essential. the sliding lathe head with high rpm and indexing capabilities are my biggest concern at the moment.