The old Barnacks are absolutely wonderful cameras and I love using them. In some ways they are more difficult because of the small rangefinder and viewfinder windows, but that just makes them a joy to work with. Based on what I understand of Barnack's intent, they are much closer then any of the later M versions.

I am very far from being an expert on anything Leica, but my Elmar 5cm/3.5 uses the A36 clamp-on filters, and they are relatively easy to find and can be had for decent prices, unlike the more modern Leica lenses and accessories. The hood I use is the FIKUS adjustable, and mine was inexpensive. It clamps on just like the filters. The filter and hood may vignette on the 35mm Elmar, but I haven't noticed that problem with my 50mm.

I do not use the Summitar or the Summaron so I cannot speak to those.