As you are probably aware, there is a pecking order among the III series cameras - the IIIa is a very good user, but collectors look down on it because so many were made. The IIIf is more expensive but may be the best user, for some reason collectors pay more for a "red dial" model than a "black dial." There are some very bad IIIc models around which were made during the war from inferior materials, you can tell these easily by the fact that the chrome finish is cracking and peeling off. The IIIg was regarded in awe by users of screw-thread Leicas as being the ultimate form of this kind of camera - as the ranks of people who thought this way become thinner with time, it has probably become less desirable, since an M2 or M3 can be had for the same money and is much more practical. I personally have owned models IIIa (2 examples), IIIb, IIIc (2 examples) and IIIf (2 examples). I enjoyed using them all but am quite happy now with a Voigtländer T. The best investment camera I had (but sold too soon) was a British Reid III, a copy of the Leica IIIc. The Reid was an inferior picture-taking instrument (fragile shutter) but because of rarity now sells at £1100 or more in mint or near-mint condition. I don't see any III-series cameras gaining significantly in value - for example, when I was a student in the 1960s, I bought a Leica IIIa with Summar lens in probably Exc++ condition for £25. This would now sell for £250 to £300, which is only in line with the rate of inflation.