Leica M Lenses
When I bought my first Leica M, a Typ240, I bought a 28mm Elmarit and 50mm Summarit. I quickly expanded to a 35mm and decided to buy the more expensive 35mm Summicron. Today, I sport a 21mm Super Elmar, 35mm and 50mm Summilux. If you are getting into Leica, a collection of Summicrons is a great place to start. they are light, easy and forgiving to focus and not too expensive. Or a Summilux. I recommend a 35mm as the range is more versatile than a 50mm.
Answering Questions about Leica
Where do the Leica Lens names come from?
Leica names their lenses based on their F Stop, or speed/aperture. Specifically Leica M lenses:
How Leica Lenses are Made
This video sums up why Leica lenses are so expensive. You can’t put a price on their craftsmanship.
I don't recommend getting a 50mm lens as a first Leica lens
Coming from DSLRs I think it is very tempting to get a 50mm as it is the prime lens size most DSLR users get. So we tend to be used to it and think it is what we want. But what new Leica lens buyers forget is that you can’t zoom in or out on the prime Leica lenses. So with a 50mm you prevent yourself from having a wider filed of view. That can be easily fixed by getting say a 28mm as well. But, I think when learning a new camera system, get one lens, see how it al works and expand from there. Hence I recommend the 35mm focal length. It does everything you need. 35mm is ideal for street photography, urban landscapes, close up or far away subjects. There is so much you can do with the frame. You can have the close, shallow depth of field look of a 50mm, but have more in the frame.