The early Dakotas weren't always the most accurate guns around. Those of more recent vintage tend to be above average. I understand the company is in flux at the moment, which might affect your decision.
The break-opens don't always have a great reputation for accuracy either, and they're not as strong as falling blocks. Extensive use with high-pressure cartridges - especially handloads assembled by the "The loading manuals are written by lawyers" types - can accelerate wear on break-open guns. Having said that, such guns can be very accurate and perfectly strong, with sane loads.
The limited experience I have had with European single-shots leads me to believe that they are at least as accurate as the typical American magazine rifle, but not as accurate as "specialty" bolt rifles meant for exceptional accuracy. You can almost certainly count on the Merkel and Blaser to be honest 1.5 MOA guns, which doesn't light any fires among the internet cognoscenti but will, in Mr. Cooper's words, do everything that needs to be done in the field. The Dakota has the best workmanship of the three, IMO, with better machining tolerances and wood-to-metal fit. The Euros are no slouches in either department, with the Blaser probably the Dakota's equal in machining tolerances. For some reason, though, the Europeans don't seem to regard wood-to-metal fit with the same importance as do the Americans and Brits. Merkel in particular likes "proud" wood, which still strike me as less pleasing to the eye.
Regarding weight, the Ruger #1 is a more-or-less copy of the Farquarson, which was originally used primarily for cartridges now regarded as on the large end of the scale. Most of the original Farkys are chambered in things like the .450 NE, which really ask for nine or ten pound rifles. So to my mind, the Ruger is a great gun in the "Tropical" guise, but maybe less than ideal for the 7x57 and the like. For a stalking rifle to be used on light-to-medium game, I absolutely understand the desire for a six or seven pound gun, and it's there that the OP's choices shine, IMO. On the other side of the coin the Dakota 10 looks kind of silly to me when chambered in something like the .450 Dakota. Each to his own, of course...
