Disclaimer: I have an M6
If you want a meter, the M6 is really the only option (I wouldn't go for an M6TTL unless you need TTL flash - they generally cost more, and the shutter speed dial turns the wrong way). So that's why I got an M6 - I've always relied on cameras with meters, and as I can't afford a new MP, an M6 was my only choice.
If I was thinking of a meterless M (and at the back of my mind I've been harbouring the thought that I might like one to carry around with my M6), I'd go for either an M2 or an M4 - I'd want at least 35mm frames, so that would rule out an M3 for me (though if I'm ever wealthy enough to afford a *third* M body... hmm).
I'd like an M2 for its original smooth brass-everything Leica engineering, and its beautiful looks. But I'd like an M4 for its quicker film loading and M6-style viewfinder frames and rewind crank. The choice would be tricky, and I'd probably go for whichever of the two I could find first at a suitable price in good enough condition at the time.
As for the price of an M6, I think you would stand a good chance of getting one for around your $850 from eBay if you're patient (though I guess you'd run the risk of it needing a CLA and blowing your budget). I was lucky to get a near-mint M6 (from an RFF member) a while ago for approx $1200, so if you're not too worried about cosmetic appearance I'd have though a slightly worn one could be had for nearer your price.
Best of luck, whichever one you go for.