Some remarks about Barnacks:
- 1) there is a RF window and a separate VF window. Both are small, but usable; the RF after and including the III is 1.5 magnified. Can scratch glasses. The VF is close to the lens, so parallax for a 50mm is minimal (compared to a contax II for example), I was pleasantly surprised.
- 2) shooting experience
The zorki 4 is closer to the M than any Barnack as it has a combined RF/VF and some have a lever wind. The Barnacks are smaller, have also separate slow speeds (two dials to turn). They are also a tad louder than a M. Of course, a M has more accurate and quicker framing with a lens in the 35-90mm range. But if you are used to manual cameras without metering, there is not a world of differences. If the camera is pre set before the shot, it is almost as quick. There is no lever wind. With a collapsible lens it can fit in a pocket without being too heavy.
Only way to find out if you like it, is to play with one. I always thought the separate RF/VF was impractical, I changed my mind, but I did not liked it when I was a beginner.
Personally, I think using a Barnack makes sense with a 50mm using the built-in VF, and with a wide (35mm or wider) with a small external VF. It is a small and light package. Adding external accessories quickly makes it as bulky as a M, and much more quirky. (but of cousre it is fun, and I have a VIOOH too 😀 )
Keep in mind that Barnacks are old, and were often very much used. Rust, weak RF, dirt in shutter, gummed slow speeds, holes in curtains are not uncommon. Bad repairs by amateurs can also add problems and are more and more encountered. I would advise to look for any clean model regardless of year/model, as a clean 1934 III is better that a rusted 1954 IIIf anyway. The price of a full cla by a competent repairman costs almost the price of the body itself (excepting collector grade items). It should cost much less than a M though : 200-250€ for a hassle-free example is possible, but you have to look for it, and probably not on eBay.
Bottom line : try it, and do not rush.