My general thoughts on the Bessa series:
With each successive model, Cosina improved the build quality of the camera, using a bit more metal and less plastic. The doors on the L and R are made of plastic. The door on the L is quite flimsy, and the body is very lightweight and doesn't inspire confidence. However, the camera has proven to be a good performer, although I still have some concerns about the long term.
The R added a rangefinder/viewfinder, and the meter display moved into the viewfinder. The camera seems a bit more sturdy.
The top deck of the L and R are made of plastic and not a high grade of plastic, from what I could tell. The secondary rangefinder window on my R has some rough edges -- should have been smoothed before painting, I think.
Still, neither top has cracked or been damaged, but then again I haven't dropped either camera.
The R2 brought the M mount. I haven't handled the R2 for any length of time, although I have the Rollei 35 RF, which is a clone of the R2. The back and top deck appears to be made of metal, although I don't know if that was just the Rollei or if it also applies to the R2.
Cosina also made two variants of the R2 (R2S and R2C) to accept either the old Nikon or Zeiss Ikon Contax bayonet lens.
The Bessa is essentially an R2 with only a rangefinder but no viewfinder. So focusing and composing occurs in two windows, for those who want to recreate the Leica TM days.
The R2a and R3a appear to be mostly metal. I haven't handled one, so I can only go by appearances.
On the L, R and R2, the silk-screened markings are very susceptible to wear. I was very gentle with my R, but some of the markings for the frame selector still wore away. I've also read of instances where the name of the camera has worn away after a short time.
The rangefinders of the Bessa cameras appear to be a weak point -- there are way too many reports of vertical and horizontal misalignment.
For their cost, these are really excellent cameras. You can get a new R + 35mm lens for $400 (from CameraQuest). That's a tough deal to beat. The Zeiss Ikon body is $1,400, while the Leica M7 (body only) is $3,495 and the LHSA MP (body only) is $4,950.
The Cosina Voigtlander lenses are probably the real steal here. Very good to excellent optically and mechanically.
Here's my own writeup of the R. It hasn't been updated in three years.
http://elekm.net/bessa-r/