I am also looking for a good 50mm lens for my Leica IIIf. I have been using the Elmar 50/3.5 that has come with it, but it flares too easily for my taste (even though it's been cleaned, but I suspect it's lost its coating). I tend to use it only in overcast days when the contrast is lower. But it's becoming too limiting.
On the other hand, I am not too inclined to get something new. I don't need high resolution and sharpness (for that I can use my Nikon stuff). I just would like something with a little (just a little) more contrast and not suffering from flare that much.
Collapsible too? An Industar-50 or -22 can sub for your Elmar or even do better. If you're looking for a non-collapsible, get an Industar-61 L/D. The I-61 L/D often are found with a rather large rear cap called FED-5.