>>4046673I see most M9's around $2500-3000 on ebay. I wouldn't buy one though, as Leica no longer repairs them and they are prone to sensor corrosion, which itself is a $1k repair through a third party.
If you're set on a digital M, aim for something newer.
>What's the best way to shoot full-frame M mount lenses these daysThe big thing to know is that there is a wide variety in thickness of sensor glass, which can lead to smearing around the edges, and is especially troublesome for wide angle lenses. Picrel is the same lens just on an M240 vs a7s. The SL series would be best in this regard, also outside of your budget.
Pretty much all the brands have various focus aids (peaking, magnification, dual display, split, etc), but vary in quality / effectiveness. At $1k, I would honestly just go for whatever has the best viewfinder.
The closest to the "rangefinder" experience is definitely an X-Pro2, which gives you an offset OVF with frame lines. It can focus with more precision (thanks to magnified ERF display), but it is a slower process. The "patch" is offset to the corner, but you also aren't locked into focus & recompose. Not FF though, so you have to get used to different focal lengths for your lenses. They are a much better experience with AF lenses.
For reference, I use an M10, and use the glass on X-Pro's (and GFX) too.