It all depends on how important it is to you to be able to use different lenses, Pedro. If you can be happy for awhile with a fast 40 or 45, get the fixed lens RF. If you think you'll want some flexibility to shoot other focal lengths, get the Bessa R as a starting point.
It's nice to have a kit with, say, a 35 and a 75. It's also true that the more you shoot with just one focal length, the better your composition will be. I believe it really depends on your tastes.
Deciding between a Barnack Leica and a Bessa R, since they're priced similarly, is more a question of whether you want an inboard meter in a new camera vs a fully manual, nonmetered camera. IMHO.