Rarely post but saw this and had to chime in. Since you are worried about nailing focus on the rangefinder with an M10, stop looking at the 35mm options and buy the Leica Elmarit-M 28mm f/2.8 ASPH. At f/8, everything from 1.5 meters to infinity is in sharp focus if you zone focus. You can find used versions well within your $1500 to $2000 budget. It is tiny, incredibly sharp, and removes the stress of patch focusing while walking.

Stumbled upon this thread today and felt like I should chime in with a few safety and practical checks, especially since you're making a pretty big jump from mirrorless. Making the switch to a rangefinder is super rewarding, but it definitely has a learning curve (and some inherent gear risks if you aren't careful). Looking at your shortlist, I'd break down the choices through a purely practical lens:

• Leica Summicron-M 35mm f/2 v4: The classic choice, but honestly, those older v4 models are getting up there in years. If you go this route, just make sure to double-check the glass for internal haze or cleaning marks under a bright light before you buy, because servicing old Leica glass can get surprisingly pricey. It handles great on the M10, though.
• Voigtlander Nokton 35mm f/1.2 Aspherical II: Way too heavy for all-day walking in my opinion — trust me on this, front-heavy setups get exhausting after about four hours on your feet. If you want Voigtlander, look at the smaller 35mm f/1.4 instead, but just be aware of the focus shift issues at close distances.
• Leica Elmarit-M 28mm f/2.8 ASPH: Probably the safest bet for weight and zone focusing, but whenever you're buying used lenses in that $1.5k–$2k range, I'd recommend getting the mount and rangefinder coupling checked out professionally if you're not entirely sure how to test it yourself. Whatever you pick before your trip next month, just make sure you practice zone focusing in your neighborhood first so you aren't stressing about the patch while traveling. Have a great trip!

Quick reply while I have a sec because choosing your first M-mount glass can genuinely stress you out (we've all been there) — I'd just recommend being super careful with your budget and condition checks before pulling the trigger. Since you're worried about the rangefinder learning curve and want something reliable for travel, here is how I'd weigh two very different directions:

• Leica Elmarit-M 28mm f/2.8 ASPH: Incredible tiny footprint, and zone focusing at f/8 with a 28mm is almost foolproof once you get the hang of it. Just make sure if you buy used, you double-check the 6-bit coding if that matters to you, and always inspect the front element for cleaning scratches.
• Voigtlander Nokton 35mm f/1.4 SC: Way lighter than the big 1.2 version, very classic rendering, and fits easily inside your budget. But wait, no — remember that older single-coated Voigtlander lenses can flare pretty intensely if you shoot into bright street lamps, so just make sure you test it in harsh light first or check with a reputable dealer who offers a return policy. At the end of the day, whatever you pick, I'd recommend getting the camera and lens combo checked by a good tech or at least practicing your hyperfocal distance in the backyard before your trip so you don't waste half your vacation fighting the gear.

Can confirm this works. Did the same thing on mine and its been solid ever since.

Start with a 35mm focal length because it matches natural human perspective best for city walking. I would recommend getting your rangefinder alignment checked out before your trip if you are buying used gear. Just make sure to double-check the focus patch calibration at a local camera shop if you aren't sure it's accurate.

TL;DR: Grab the Leica Elmarit-M 28mm f/2.8 ASPH and dont look back. I was stressing over this same exact choice last week and honestly, if youre worried about nailing focus, 28mm is a life saver. The depth of field is so much more forgiving than a 35mm f/1.4. I spent hours reading forums and realized that for candid travel stuff, you dont wanna be fiddling with the patch when the moment is happening. The Elmarit is tiny, razor sharp, and stays out of your way. The Voigtlander Nokton 35mm f/1.4 II is cool and cheap, but the focus shift drove me crazy during my testing. Just go wide, zone focus if you have to, and enjoy your trip. Youll be way less likely to miss the shot. Seriously, keep the rig light or you wont even wanna carry the M10 after a few hours...

Did this last week, worked perfectly

Building on the earlier suggestion, this comes up a lot — I switched to a wide prime years ago and never looked back once I stopped stressing over the patch.