The main issue is screen size and iOS limit. Also RAM size and screen size and resolution.
I won‘t use an iPad mini since the size is too small for sheet music.
I also won‘t use a version 1 iPad because of the limitation to iOS 5, the iPad 2 can get a bit slow and the screen resolution could be better, but it works and I have two of them.
The iPad 3 has a better screen but for larger scores the RAM can limit the use (about 25 pages per title, iPad 2 was about 40). At least with the current version of Calypso. But for lead sheets, fake books it works. But they are limited to iOS 9 and that can be a problem for some apps, now or rather soon.
On the other hand they can be very cheap used.
It depends on the size of your music library, but I prefer 64 GB (or larger) and won‘t get lower than 32 GB. Also to keep some free space (about 3-4 GB) or the iPad can get slow. Also limit the number of apps on this iPad. Some can slow it down.
For conductors and maybe also musicians that need to play from longer parts or scores a double page view might be more pleasant to use, but then you are quickly at the largest iPad you can get.
A page turner is a need, either a commercially available one or building one from a cheap (Bluetooth) keyboard. There are nice examples on YouTube. But not every app can select the keys for page turning.
Try to get a silently switching one. Simple stomp box switches are loud!
So, that‘s from the cheap side of digital music reading.