Re: Best years for OMC 10 horse?

[Don]

Tough question for sure, let’s assume for this discussion that you have no personal preferences for brand or paint scheme.
The 55-57 models have the larger/more durable/easier to find parts for gearcase. They say the small gearcase is faster, maybe, but is that really worth the reliability difference? The main thing that killed the smaller gearcases was water intrusion, which caused the lower housing bushing and driveshaft to wear. I agree with Chris, the smaller gearcase can be reliable, provided oil level and condition are monitored frequently.
The other issue on these engines is wrist pin wear. There were no actual wrist pin bearings on these 50s models, so extended high speed/lean oil mixtures will cause wrist pin/piston and rod bore wear. This problem was solved in late 1960 models with a larger diameter wrist pin and caged needle bearings. But, it sounds like you want to stick with 50s models. Mumbles has found that the 59 models had beefier wrist pin bushings, and I really like the 59 Johnson styling and graphics, the Evinrude bread box models are unappealing. The 59 models also had a thermostat, a plus for extended trolling. 1960 models introduced fuel pumps and got rid of the pressure tanks.
So, not really able to give an actual recommendation, just pointed out some differences. I like your idea of a 57 Evinrude or Johnson model, just be sure to check carefully for wrist pin slop.