I shot with a III for while and it's standard screen was terrible. If you're going to shoot with one regularly I'd say a screen upgrade is essential.
I read the mirrors of the pre ww2 cameras deteriorate easily, and that they used glass screens. The one I’m playing with is from ~ 1960. The screen (og plastic) isn’t fresnel (so the corners are dark) but it’s alright to focus. Better than a Hasselblad. The mirror is perfect but the screen has some cleaning scratches that can get distracting if using the loupe.
I’m finding focusing with my left hand, whilst my shutter finger down by the lens really weird. Otherwise ... I’m quite liking it so far.