Seeing that you already have a Nikon system, you might want to explore a higher-priced rangefinder system. If you don't want to spend a fortune -- yet or ever -- you can try the Cosina Voigtlander bodies and lenses, which give you the interchangeable-lens rangefinder experience on a budget.
The Cosina bodies have very nice viewfinders. Make sure that the rangefinder alignment is good.
If you buy used, you probably can pick up a used body and two lenses for a fraction of the cost of a single new Leica lens (or certain Zeiss lenses).
Of course, a Cosina isn't a Leica, although they are very good bodies.
If you really enjoy rangefinders, you can continue to build out your system or move up to the higher-priced gear.
And if you don't enjoy the experience, you probably can sell the gear for the same price you paid and get out without a loss.
But find what works best for you.
Lately, I've been using SLRs with a sidetrip to an Argus A2B an several other viewfinder cameras. Still love my Contax IIA.