If TTL is not an issue, (would be for me!), the M4 is the last all mechanical Leica. I don't believe it has a frameline for 28mm, 35mm would be the widest lens you could use with it. For the next model, (technically the M5, but that had a lot of transitory technology and is larger than a classic M) the M4-P, production moved from Germany to Canada, viewfinder modified to frame as wide as 28mm, but rangefinder patch can flare and make it hard to focus, sometimes in side lighting. M6 had TTL metering and is worth consideration, drawback is that the older ones are getting long in the tooth and may need an expensive replacement of the metering circuitry before long. BTW if you have a serial number for camera or lens you can check the Leica website to determine the year of manufacture.
The M6 TTL (named for inclusion of TTL flash) is the first model I'd consider. It's sure to be new enough that the electronics are good for a while. I'd recommend the .72 finder magnification for all frames from 28mm to 135mm. It a good compromise. All of these models are subject to rangefinder flare.
The current production models are the M7 (has electonically controlled shutter and shutter priority auto exposure capability) and the MP a kind of retro look and is high priced relative to features. It's all mechanical except for the meter.

I own an M6TTL and really love it. If I were buying used this one would be at the top of my list in terms of bang for the buck. You can expect to pay about $1200 for a good one. the M4 has some appeal, too, as the last of the mechanical (no meter) German M's.
Take care,
Tom