I tend to analyze a lot before purchasing - and I was trying to compare a rangefinder to SLR's - so completed a list and fleshed it out with a R6 entry for you (I know you didn't ask, but I already had 80% of it, so what the heck! )
I hope this helps you - I decided to include it because you mentioned you were concerned about weight and size of the camera!
Size and weight (w/o lens)
Leica R6: 148 x 97 x 57mm : ~627g (web search)
Leica M7: 158 x 101 x 62mm: ~890g (web search)
Leica MP: 158 x 101 x 62mm: ~790g (Web search)
Ziess Ikon RF: : 138 x 78 x 32mm: ~500g (Zeiss website)
Nikon F2A: 152 x 102 x 65mm: ~840g (web search)
Nikon FM2: 142 x 90 x 60mm: ~540g (FM2/T is ~515g I think) (Nikon site)
Nikon 50mm/f1.8 AIS F-mount lens weight: ~210g (weighed mine on scale)
Leica 50mm/f2 SLR R-mount lens weight: ~290g (web search)
ZI 50mm/f2 RF M-mount lens weight: ~210g (Robert White's site)
Not counting UV filter and lens cap.
So a kit might weigh:
Nikon F2A with 50mm: 985g
Leica R6 with 50mm: 917g
Nikon FM2 with 50mm: 750g (confirmed appx right with kitchen scale)
For comparison (I did mine in reverse - I was considering the below):
Leica MP + 50mm:1000g
Leica M7 + 50mm: 1100g
ZI RF + 50mm: 710g
You can see that the F2A isn't too bad compared to Leica rangefinders, but if you want a similar experience, the FM2 is a light weight camera (and when I got the ZI package with a 35/f2 Biogon, my wife was surprised they felt about the same weight, and while the FM2 was a bit bulkier, they all handled appx the same! :o )
You probably know what you want already - so you should get that, but if you want some additional data to think about, I hope this helps!
Also - the focussing directions won't be an issue - though there will be some new habits to be learned - but since each person is wired differently, YMMV.
Good luck and tell us if you decide anything!