#1 Have you asked Fred about 2525 balls crews because I've had them from him and he's advertising them on his site.?

#2 Those sizes will be fine.

#3 Wiring is no different for any servo, they all pretty much wired the same way. The servo tuning is most important but unfortunately Low cost and simple auto-tuning drives don't really go hand in hand. If you want easy tuning then you need drives which autotune and these tend to cost more. Look For Delta B series drives which are supposed to be good auto-tuning drives.

Why do you want Servos.?

#4 No, now with 800mm you have gone into 2 ball screw territory. The single screw will work but you will have to reduce the DOC/Feed at the outer edges.