Regarding accessory finders...
If you want a single-purpose finder, the new C-V are probably the best buy.
If you think you might be getting other focal-legnth lenses for the III, you might want to consider keeping your eyes open for some sort of multi-focal finder.
In my humble opinion, the Leitz "VIDOM" and "VIOOH" Imarect finders are more valuable as collectibles than they are useful accessories; others will disagree with me, but I think there or more user-friendly finders out there.
My top-picks for multi-finders are:
1) Nikon "Varifocal" ( not to be confused with the "Variframe")
2) TEWE Polyfocal (German copy of the Nikon)
3) Zeiss "turret"
4) Russian copy of the Zeiss turret finder.
Of the four types I listed, the TEWE and Russian turret are probably the least expensive.
The great thing about the TEWE and Nikon vari/polyfocals is that the frame size of the viewing window remains constant as you dial-up the different focal legnths; with the Leitz finder, the viewfinder image gets smaller and smaller as you dial-up longer focal-legnths. By the time you set it for 135, the vF image is tiny. Accurate, perhaps, but not terribly easy to use.
You might find a 35mm folding "sports finder" a bit cheaper than the optical finders...
Good hunting !
Luddite Frank