Everyone else has covered the economic practicalities of spending $6500 on a new camera, so this is my story of digital Leica acquisition.
In early 2010, after the huge buildup of the M9 announcement, I finally bought one brand new. It was AUD$9300 and I felt like my guts had fallen out of my ass. It was the single most expensive thing I had ever bought in my life, including a car. That's how big it was for me.
It turned out to be the best and most satisfying photographic decision I've ever made. Up until then, I had shot with compacts and DSLR's, as well as a Zeiss Ikon. I loved the IQ of the 5D Mark II but not the size and weight; I loved the size and handling of the Ikon but not the cost and inconvenience of film. The M9, therefore, was an utter revelation.
I've sometimes had second thoughts about buying nonessential items, thinking, 'If I hadn't bought that, I'd have that money to be able to do X'. But I have never, ever had that thought about the M9.
You already have a D810 with Sigma Art primes, and yes, that combination will definitely give you image quality at the level of the M10 and beyond. But it doesn't give you the physical experience of shooting with a DRF that gives you image quality within the realm of the D810.
At the end of the day: there is a difference between being able to pay for something, and being able to afford something. You may be able to pay for the M10, but can you afford it? If not, then put that money back into your overall living budget and just rent a M10 to discover how you like it. I was lucky with the M9 because of my particular wants and needs; your mileage may vary.
