Dont overlook the 50/3,5 Elmar in collapsible M-mount. I have found that it outperforms the 50/2,8. It would also be a nice match with early DS M3. The 2,8 is a good lens, but i find that it is a bit soft at 2,8. The 50/3,5 is sharp already at 3.5. The other alternative is the 50/2 DR even though it might be a couple of years "younger" than the M3. If you can find a clean one it will hold its own against even a late 50/2 and with the goggles it will work well as a "mid-range " macro lens. It is also an example of Leica build quality that most likely has not been surpassed yet. Check that the goggles go on smoothly as on early DR's they can give you problems. Also check for de-cemented prisms in the goggles. Once you have that kit, look around for the 90/4 Collapsible - another example of Leica build quality at its best!
I have never been a great fan of the collapsible Summicron. Usually soft at f2 and mediocre performer for the rest. They also had very soft coatings on the front element and to find a clean one is not easy. Also can suffer from "sticky" aperture ring to boot. Have a great time using the M3.