Re: help me decide; sell X100 for Leica yes/no?
I'd take the M2 over the M3, the M2 viewfinder covers 35/50/90 lenses whereas the M3 is 50/90. Also the M3 viewfinder can fail very expensively. But it's all down to personal choice at the end of the day.
As for the Jupiter, yes if you find a good example, but you can also find screw mount 5cm Elmars for not a lot of money, as well as several other options.