Before jumping in and spending a lot of money on a new camera and lens, you should try someting on the less-expensive side. There are literally hundreds of kind of fixed-lens rangefinder cameras, and some are stellar performers. The Olympus SP, Canonet QL17 GIII, or a Yashica Electro will help you learn the basics, and are relatively cheap. FSU cameras are hit-and-miss, with the emphasis on miss. I have bought several, and not a single one has worked properly out of the box.

Leicas and Nikon rangefinders are nice, I have several of each, were you in Japan I could loan you one to try out. Nice as they are, they aren't going to outshoot your Nikon, which is built to a higher standard of sophistication and quality. Rangefinder lenses have an advantage over SLR retro-focus lenses, but 99.9% of people would never notice the difference.

The advantage rangefinders have over SLR cameras are a slightly smaller size, and a quieter shutter. I shoot either type regularly, and though I prefer rangefinders, there is nothing wrong with shooting an SLR.