Thanks to everyone for the very helpful advice! Just to give an update - the outfit I'll be getting does come with a focusing screen and a dark slide for the A12 film back. I will have to get a lens cap, though.
I think that my first item for a wish list will be different film backs. I like the idea of switching between color and black & white, but also between negative film and slide film. Or I may start off with a lens hood, which is less expensive. Replacing the focusing screen is the last item on the list, since I've been dealing with a crappy viewfinder on my TLR and I have learned my way around it.
Oh yeah, and I'll have to get more lenses at some point........