I have a question. I see you listed a IIIc and a III. I'm not aware of a Retina III; a IIIc, a IIIC, and a IIIs, but not a plain old III. Is there a missing letter as regards this Retina?
Over the years I've owned a few Retinas: two or three IIc's, a IIa, a II (014), and a couple Ia's. The IIa and the II are little bigger than the I series and besides the rangefinder you get a fantastic Schneider Xenon or Rodenstock Helioplan lens. The advance lever of the IIa (and Ia) engages a shutter cocking rack that can wear out prematurely. This gives the II an advantage. On the other hand, the plain II has an uncomfortably long shutter button action; not like the more precise shutter release on the IIa. They're built to a very high standard. So are the C series, and the C series is possibly finished just a bit better: better chrome and the fake leather holds up better than the real leather on many of the earlier cameras. All tend to fall prey to yellowed viewfinders. The IIc has a wonderful feel in the hand, like a serious camera. You would probably get used to the bottom advance lever. It never bothered me. The IIc cameras I've owned aways seemed like a great compromise between pocketable size and real camera handling. Same wonderful lens, only a stop slower on the IIc. The C series also allows the use of add-on lenses in 35mm and 80mm lengths. With these lenses one has to use the rangefinder more like an uncoupled rangefinder. Not convenient, but the smaller, 5.6 35mm is useful as a scale focused accessory lens to carry along.
If small size is paramount go with the II or IIa. If you think you might want to get hold of the 35mm (forget the 80mm) or you just want a better handling package (admittably subjective) go for a C series. That's my advice. But, beyond all that is the question of condition. These are all elderly citizens of cameradom so take that into consideration. A CLA may be in order for any of them.
Randy