To run quickly through your list.

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)

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.

Lathe ATC. Doable. Plenty designs around.

Stock puller. There are a few simple designs around. Or you could go all out with a bar pusher :)

Tailstock. That will depend on what you're wanting to support. Big bars, pneumatic pressure will likely do. Smaller stuff, you'll probably want more accurate positioning.