Interestingly, IIIg prices seem to have taken a bit of a tumble recently and there there have been bodies in very good condition advertised for £500 to £600 in the UK recently. (I seem to recall that production numbers were something like 40,000.)
I'm no expert on the camera but I can share a few experiences about the IIIg that I inherited from my father. He bought this from a very well known UK dealer as an "Unusually mint Leica IIIg" for a premium price and with hindsight, "Unusual" was probably the best description! From the outset, it had a problem with the rangefinder which stopped moving below about 8 feet. This was put right under warranty although the problem quickly appeared again. After my father died, the camera passed to me and I decided to have it looked at by Leica guru Malcolm Taylor and although he assured me that it could be fixed, it contained a catalogue of disasters! Firstly, the rangefinder mechanism had been assembled incorrectly (one part was apparently even upside down!) and although it worked properly when reassembled, as soon as the top plate was fitted, the parallax correction stopped working (note: the framelines only move upwards and downwards in the IIIg and not towards the corner as in the M series). This turned out to be caused by a slight depression on the top plate which you can only see if you know it's there! The rangefinder mirror (a common poblem on all of the screw mount Leicas) also needed replacing as the rangefinder patch, although usable, was dim.
Then the shutter. This was giving excellent, accurate exposures but it had apparently had new curtains at some point in its life and these were not positioned correctly. After sorting this out, it still took a couple of attempts before he could get the "T" setting to work properly. Apparently, this needs very fine adjustment to ensure that it stays open.
The result? Well, it's now a (near!) mint IIIg which is an absolute joy to use. Yes it is bigger than the IIIc/f but the larger and brighter viewfinder makes up for that.
So the bottom line is, beware the CLA hackers where expensive cameras like this are concerned. If you can get a potential purchase checked over by a genuinely experienced Leica repairer (rather than one who says that they are!), then it's certainly worth it. Simple advice would be to carefully check the rangefinder (and parallax correction) condition/operation and the shutter operation as best you can. If anything is less than silky smooth, be suspicious.